Disclosure; I have only seen this demonstrated one time - never owned an airhead with rear disc brakes.

Remove the caliper, block the caliper with an object to take up the space where the disc would otherwise be located. Hold or temporarily attach the caliper in a position that is well above the master cylinder. Proceed to attempt to bleed the system.

Alternatively, you might conduct a modified bench bleed where you remove the caliper and carefully bleed the system until brake fluid is established to be present at the caliper end of the hose. Then replace the caliper and proceed as described above.

My 1979 has the same problematic rear brake. Works great, but man, oh man is it complicated. Complicated to build and complicated to work on. So if it's any consolation, I'm there with you, brother !

Several thoughts and questions come to mind:
► It's unclear why this work was undertaken. Was the rear brake operable BEFORE this work commenced ? Or is the bike being brought out of long term storage and the rear brake was questionable... which necessitated the hose replacement ?

► DOT4 brake fluid is very strange stuff, and in long term storage it turns to a thick varnish-like substance which will grow to as much as 1/8 inch thick and clog small passage ways. (Like the small orifice where fluid enters the compression chamber !) This is why BMW tells you to replace DOT4 fluid annually. This is one reason why I changing over to DOT 5 Silicone brake fluid.

► Even if the m/c is in good working order, the compression chamber will not fill with fluid from the reservoir until the piston returns to its "home" position. That could be becasue of the external push rod adjustment, OR caked-on brake fluid blocking the piston movement internally.

► It's unclear which type hose you installed. Some makers provide a long hose that runs from the m/c all the way to the caliper (replacing the steel line and the flexible hose), and other makers provide a short hose that simply replaces the OEM rubber hose at the caliper. Can you please be more descriptive ?

► If you used the full length hose, then the connection at the m/c (which was straight into the steel line) is replaced with a banjo fitting into the hose at the output of the m/c. That new banjo fitting could be the problem. You might wish to inspect the banjo bolt to make sure the 2 drilled holes interconnect. You can't use any old bolt at that location; it must be a drilled banjo bolt.

► I find that access to compressed air is almost a requirement to work on brake systems. Air is a great way to test fluid paths through hoses and fittings. Generally, anywhere air can go then fluid can also pass, but air is a LOT less messy, especially if disassembly is required. Compressed air can also blast though blockages caused by old brake fluid.

Having said all that, here is my suggested sequence of testing:
1) Loosen the hose connection where the reservoir enters the m/c. Make sure you have fluid coming down to the the m/c.

2) Remove the hose or tube from the rear of the m/c at the output. Lay your finger tip over the output and use it as a one-way valve. VERY slow motion pumping (one cycle every 4 seconds or more) of the brake pedal should cause fluid (or air pressure) to be pushed out of the m/c and past your finger.

3) If there is no fluid or pressure detected, then unscrew the barb where the fluid enters from the reservoir. With the brake pedal depressed as far as possible, insert a sewing needle down into the fluid entrance and find the small hole. Wiggle the needle around in that hole so as to clear it. Reassemble and go back to step #2.

4) Once fluid is being pumped out of the m/c, then we reconnect the line and move to the rear brake caliper. The best way I've found is as Mr 8053 suggested. Remove the caliper from the bike, and having separated the 2 brake pucks with a 3/8" thick chunk of wood, lift the caliper so that the bleed nipple to be the highest point in the rear brake system. In this way air will naturally be chased out of the system. I always bleed my rear brake the "old fashion way", with a second person applying SLOW 4-5 second brake pedal pumping pressure, while I open the bleed nipple. Initially you may need to open the nipple and simply hold your finger over it and repeat the "one way valve" routine.

5) It is IMPERATIVE that as soon as the brake system is working, reassembled, and sealed, that the exterior of the bike be washed with hot soapy water. If not, everywhere DOT 4 brake fluid touched will be without paint within a dozen hours. One more reason I use DOT 5 Silicone brake fluid.

HINT: If you remove ~2mm of thread from the caliper mounting bolt hidden by the swing arm, then you can remove the caliper without having to mess with the rear axle or caliper mounting plate. Shortening that one bolt makes the entire job MUCH simpler !!

Hope this helps !

Thanks, you guys for replying - I have no doubt the solution will be found here.
I will get to this Friday afternoon and post results over the weekend.
Cheers!

For starters, thanks both of ya guys for taking the time to help a brother out.

► It's unclear why this work was undertaken. Was the rear brake operable BEFORE this work commenced ? Or is the bike being brought out of long term storage and the rear brake was questionable... which necessitated the hose replacement ?

There was no indication of a problem beforehand. I've been going through the bike and the old rubber hose is at least 8 years old, and may well be the original, so I replaced it with a Stainless (Spiegler/short) hose.

► Even if the m/c is in good working order, the compression chamber will not fill with fluid from the reservoir until the piston returns to its "home" position. That could be becasue of the external push rod adjustment, OR caked-on brake fluid blocking the piston movement internally.

After replacing the line and master cylinder reservoir, pedal had full range of travel with no hydraulic resistance noted and system did not pump up.

► It's unclear which type hose you installed. Some makers provide a long hose that runs from the m/c all the way to the caliper (replacing the steel line and the flexible hose), and other makers provide a short hose that simply replaces the OEM rubber hose at the caliper. Can you please be more descriptive ?

It's the short replacement - the original pipe is still in place.

1) Loosen the hose connection where the reservoir enters the m/c. Make sure you have fluid coming down to the the m/c.

Yep.

2) Remove the hose or tube from the rear of the m/c at the output. Lay your finger tip over the output and use it as a one-way valve. VERY slow motion pumping (one cycle every 4 seconds or more) of the brake pedal should cause fluid (or air pressure) to be pushed out of the m/c and past your finger.

Hmmm...Not a steady stream of fluid as I'd like...Even after pumping for quite a while, no consistent pressure. I'm assuming fluid is escaping around piston seals unless you've got other ideas...Some master cylinders are not worth rebuilding (but this one's $150.00) Ya guys had any luck rebuilding these?

HINT: If you remove ~2mm of thread from the caliper mounting bolt hidden by the swing arm, then you can remove the caliper without having to mess with the rear axle or caliper mounting plate. Shortening that one bolt makes the entire job MUCH simpler !!

I'll definitely do this while I'm waiting for parts.

Hope this helps !

Yes! Now, can anybody tell me how to disassemble this master cyl? I don't see a circlip or a snap ring I can get to.

2) Remove the hose or tube from the rear of the m/c at the output. Lay your finger tip over the output and use it as a one-way valve. VERY slow motion pumping (one cycle every 4 seconds or more) of the brake pedal should cause fluid (or air pressure) to be pushed out of the m/c and past your finger.

Hmmm...Not a steady stream of fluid as I'd like...Even after pumping for quite a while, no consistent pressure. I'm assuming fluid is escaping around piston seals unless you've got other ideas... Some master cylinders are not worth rebuilding (but this one's $150.00) Ya guys had any luck rebuilding these?

You have air in the system. You need to get ALL the air out of the system, or it will not behave as intended. We do this by placing our index finger over the fluid port to form a one-way valve that allows fluid out, but DOES NOT allow air back in.

But first you need to make sure the master cyl is performing. If it performed BEFORE you did the hose replacement, it should work properly now. This is a detail you haven't bothered to share.

1) With the steel line removed from the rear (output port) of the master cyl, and your index finger covering to seal the port, you should get a squirt of brake fluid every time the brake pedal is depressed. When the pedal is up, brake fluid should rush in from the reservoir to refill the compression chamber. And you should get another squirt every subsequent time you depress the brake pedal. The amount of fluid in each squirt is going to be slightly less than one tablespoon. While the finger is still in place, remove all pressure from the brake pedal.

2) Replace the steel line on the m/c.

3) Remove the caliper and place a piece of wood between the pucks. Remove the bleed nipple from the caliper AND physically raise the caliber so that the bleed nipple hole is the HIGHEST point in the system. Now, place you finger over the nipple hole to once again form a one-way valve. Get an assistant to SLOWLY depress the brake pedal and then allow it to rise again in a 4-5 second cycle. Do this as long as you hear air spitting out from under your finger. DO NOT allow the reservoir to go empty.

4) When you stop hearing air leave the system, remove all pressure from the brake pedal. Replace the bleed nipple and snug it up. With the caliper in the same position (bleed nipple at top) you can now start final pressure bleeding as per your shop manual.

Best of luck.

Yep, at the very least, I'm going to order rebuild kits, but I want to get a look at the interior surfaces of the master cyl and calipers to see if that's even worth doing...Problem is: I don't see a circlip or a snap ring on that master cylinder.
Clymer says something about pushing a small diameter rod into the brake line orifice and pushing out the internals...
I have not been able to pull this off by hand.
They didn't say anything about using a press.
Anybody ever had one of these apart?
Sure like to order parts early in the week!

You have air in the system. You need to get ALL the air out of the system, or it will not behave as intended. We do this by placing our index finger over the fluid port to form a one-way valve that allows fluid out, but DOES NOT allow air back in.

Right. I get that.

But first you need to make sure the master cyl is performing. If it performed BEFORE you did the hose replacement, it should work properly now. This is a detail you haven't bothered to share.

I did not perform this before hose replacement.
(This was an oversight on my part, not an intentional omission with which I decided not to be bothered. I'm striving for a balance between brevity and detail.)
When I released the pedal with my finger still over the opening, the piston returned to its home position, which is why I assumed fluid was getting past the seals.

1) With the steel line removed from the rear (output port) of the master cyl, and your index finger covering to seal the port, you should get a squirt of brake fluid every time the brake pedal is depressed. When the pedal is up, brake fluid should rush in from the reservoir to refill the compression chamber. And you should get another squirt every subsequent time you depress the brake pedal. The amount of fluid in each squirt is going to be slightly less than one tablespoon.

Yep.

While the finger is still in place, remove all pressure from the brake pedal.

2) Replace the steel line on the m/c.

Can you please clarify this one: I have to remove my finger to replace the steel line - hence loss of one way valve. Shouldn't I keep the pedal depressed until the line is replaced and the system is closed?

I've got the system apart, and knowing DOT 4 does funky things as it ages, I think it's prudent at this point to disassemble the mc and caliper to inspect the surfaces and re-seal if ok.
But I do have another question related to disassembly of the master cylinder - there is no visible circlip or snap-ring holding the thing together. Microfiche from Max BMW etc do not show one either. Anybody ever done this?

But first you need to make sure the master cyl is performing. If it performed BEFORE you did the hose replacement, it should work properly now. This is a detail you haven't bothered to share.

I did not perform this before hose replacement. (This was an oversight on my part, not an intentional omission with which I decided not to be bothered. I'm striving for a balance between brevity and detail.)

► And we are still in the dark ! Lot's of "brevity" and not many useful details.

► Question: Was the rear brake working before you decided to replace the hose ? Did you ride the bike and did the brakes work ?

► Question: Have you ever worked on a hydraulic brake system before, either on a car or motorcycle ? What was the outcome of that work ?

When I released the pedal with my finger still over the opening, the piston returned to its home position, which is why I assumed fluid was getting past the seals.

► Question: How can you see inside the m/c to know the PISTON returned ? Don't you mean the pedal returned ? Both the piston AND the pedal have discrete return springs. The pedal can return to its 'rest' position without the piston doing anything. Their actions are independent of each other.

The amount of fluid in each squirt is going to be slightly less than one tablespoon.

Yep.

► "Yep", you agree with the statement in theory, or "yep" fluid is physically being pumped out ?

While the finger is still in place, remove all pressure from the brake pedal.
2) Replace the steel line on the m/c.

Can you please clarify this one: I have to remove my finger to replace the steel line - hence loss of one way valve. Shouldn't I keep the pedal depressed until the line is replaced and the system is closed?

► This is the simplest plunger pump imaginable. 1) When the piston is actively retracing toward the 'rest' position is the only time air can be drawn into the system. (That is why we cover the orifice with the finger.) 2) Even with no finger, air CANNOT enter the m/c if the piston isn't moving. These 2 truths are self-evident. 3) Therefore if we simply allow the pedal to go to the 'rest' position, then we free up one hand that would otherwise be needed to hold the pedal down. If the logic of these 3 statements isn't intuitively obvious, then you REALLY ought to leave hydraulic brake work to someone else. I don't say that to belittle or be mean spirited, I say that becasue your life is in danger without properly working brakes. And these 3 facts should have been innate, foundational knowledge, the basis for the entire undertaking.... not something to be discovered along the way.

I replaced the rear brake hose and the fluid line from the reservoir on my 1979 RT. I may have had a slight advantage in that my caliper mounting bolt was already shortened by 2mm, but the whole, entire job was completed in less time than the first reply took. This is usually simple, straight forward work.

I've got the system apart, and knowing DOT 4 does funky things as it ages, I think it's prudent at this point to disassemble the mc and caliper to inspect the surfaces and re-seal if ok. But I do have another question related to disassembly of the master cylinder - there is no visible circlip or snap-ring holding the thing together. Microfiche from Max BMW etc do not show one either. Anybody ever done this?

Yes, I have done this work. But forgive me once again to point out with all due respect that if replacing the hose gives you issues, then you ought not to be rebuilding master cylinders. That job simply requires more tools, more time, more finesse, and more technical understanding.

Again, if the rear brake system was working before the hose replacement was initiated, then the m/c is NOT at fault. There is simply zero correlation between removal of a brake hose and failure of a m/c. And you have still not told us if the orifice feeding fluid to the piston chamber from the reservoir is clear.

All the best, and good luck.

Mr. Wobbly:

Somehow, it appears I have annoyed you. This was purely unintentional, and I'm honestly not sure what triggered it.
I have been polite and have acknowledged a desire to know more. I have thanked you and others for contributing and have offered further information when it has been asked of me (yes...read on).
I'm not going to spend too much time deconstructing your assertions, but I will answer a few since it looks like you've made some judgements without having read and understood my polite posts.

But first you need to make sure the master cyl is performing. If it performed BEFORE you did the hose replacement, it should work properly now. This is a detail you haven't bothered to share.

Actually, I did in reply #3518, but you missed it

I did not perform this before hose replacement. (This was an oversight on my part, not an intentional omission with which I decided not to be bothered. I'm striving for a balance between brevity and detail.)

► And we are still in the dark ! Lot's of "brevity" and not many useful details.

An unnecessary jab, if you had read the post.

► It's unclear why this work was undertaken. Was the rear brake operable BEFORE this work commenced ? Or is the bike being brought out of long term storage and the rear brake was questionable... which necessitated the hose replacement ?

To which I politely replied:

"There was no indication of a problem beforehand. I've been going through the bike and the old rubber hose is at least 8 years old, and may well be the original, so I replaced it with a Stainless (Spiegler/short) hose."

Question: Was the rear brake working before you decided to replace the hose ? Did you ride the bike and did the brakes work ?

See above. I remember the question. It appears you missed the answer.

While the finger is still in place, remove all pressure from the brake pedal.
2) Replace the steel line on the m/c.

Can you please clarify this one: I have to remove my finger to replace the steel line - hence loss of one way valve. Shouldn't I keep the pedal depressed until the line is replaced and the system is closed?

This was not intended to challenge you, Mr. Wobbly. I requested clarification based on a conflict between what you wrote and my prior experience with hydraulic brake systems, which until now, does not include airhead brakes. That was why I came here, to Airheads Beemer Club to humbly ask for some input.

This is the simplest plunger pump imaginable.

Yet here is your first response in this thread:

My 1979 has the same problematic rear brake. Works great, but man, oh man is it complicated. Complicated to build and complicated to work on. So if it's any consolation, I'm there with you, brother !

1) When the piston is actively retracing toward the 'rest' position is the only time air can be drawn into the system. (That is why we cover the orifice with the finger.) 2) Even with no finger, air CANNOT enter the m/c if the piston isn't moving. These 2 truths are self-evident. 3) Therefore if we simply allow the pedal to go to the 'rest' position, then we free up one hand that would otherwise be needed to hold the pedal down. If the logic of these 3 statements isn't intuitively obvious, then you REALLY ought to leave hydraulic brake work to someone else. I don't say that to belittle or be mean spirited, I say that becasue your life is in danger without properly working brakes. And these 3 facts should have been innate, foundational knowledge, the basis for the entire undertaking.... not something to be discovered along the way.

You've gone to a lot of effort to insinuate my incompetence, but I'm not biting. Perfectly comfortable with my mechanical abilities and achievements. If this prompts you to take another swipe, I'd invite Airheads Beemer Club to examine what it means to be a Moderator of a club with paying membership. This is not representative of the Airheads Beemer Club I joined years ago.
And Wobbly: Your behavior is not that of an expert or a leader of any kind.

You go on to say:

I replaced the rear brake hose and the fluid line from the reservoir on my 1979 RT." ..."but the whole, entire job was completed in less time than the first reply took. This is usually simple, straight forward work.

Again I point out your first friendly post:

My 1979 has the same problematic rear brake. Works great, but man, oh man is it complicated. Complicated to build and complicated to work on. So if it's any consolation, I'm there with you, brother !

I'm resisting the urge to trade insults with you.
You decide for yourself if you feel good about all this.

Webmasters, Administrators, how do you all feel about this?
I reflect on Club Canons 3 and 8. Amen.

So, I asked for a little more input, hoping someone a little less crusty will chime in:

I've got the system apart, and knowing DOT 4 does funky things as it ages, I think it's prudent at this point to disassemble the mc and caliper to inspect the surfaces and re-seal if ok. But I do have another question related to disassembly of the master cylinder - there is no visible circlip or snap-ring holding the thing together. Microfiche from Max BMW etc do not show one either. Anybody ever done this?

Still hoping for an answer on this one...anybody?

Yes, I have done this work. But forgive me once again to point out with all due respect that if replacing the hose gives you issues, then you ought not to be rebuilding master cylinders. That job simply requires more tools, more time, more finesse, and more technical understanding.

Ahh, I bet you felt good writing that. It was uncalled for, but I take no offense because you've revealed yourself to be an impotent, angry little being.

And you have still not told us if the orifice feeding fluid to the piston chamber from the reservoir is clear.

Actually, I addressed this in the very first post when I said;
"Traced clear lines all the way back to the master cylinder.
Traced clear lines from reservoir to master cylinder..."

(Edit: OK fair enough: I just re-read that and you're right Wobbly: I did address the hose, but not the orifice. However it seems like clearing a clogged orifice with a sewing needle will push a chunk of something into the cylinder. I still think a better idea would be to disassemble, clean, inspect and rebuild if feasible.)

May you be well, Wobbly.

I invite you to detach from this thread because I'm not interested in arguing with you. I don't have time for it.

I'm hoping there's an airhead or two who wants to have a polite and friendly exchange about disassembling a master cylinder for a 1978 R100S.

Sir, I sincerely apologize if I have offended you. I assure you that while I have indeed reached the point of exasperation in the 4 hours I have spent working on your problem, it is with the inexactness of the language being used and not you personally.

Case in point, "Traced clear lines...". In my understanding of this terminology you can 1) "trace" with a No.2 pencil, or you can 2) take your finger and run it along the outside of the brake line between the master cylinder and the caliper, as in "tracing" the route. Neither of those definitions answer the simple question as to whether fluid is moving from the reservoir through the master cylinder. And if fluid refuses to be pumped through the m/c, then bleeding the system is out of the question. So my frustration stems from the total absence of a single, clear phrase, "Yes, fluid can be pumped".

Allow me to summarize all the main points, so that you can get on with your repair:
• The secret is to remove and raise the caliper
• The shortcut is to shorten the lower caliper mounting bolt by roughly 2mm
• Everything you know about hydraulic brake systems (car or motorcycle) applies. There is nothing special about R100 rear hydraulic brakes. While the physical mounting and actuation is complex, the internal hydraulics are stone ax simple. Other than the special caliper position, every procedure done to bleed car brakes applies here.

All the best.

No problem Wobbly,
I understand your exasperation since you spent twice as much time on it as I did.
Especially since you thought I was troubleshooting my brakes with a writing implement. Gotta watch my use of metaphor.
Anyway, I appreciate your apology: No harm, no foul.

Just getting back to it and I'm glad I decided to take a closer look.
Master cylinder disassembly was harder to find a procedure for than actually doing the job:

No circlip.
No snapring.

Just take a small rod and drive the piston out the same way it went in - see the pic (repeated attempts to upload the correct pics but getting duplicates...Will try again later.

I used a small round file from a Harbor Freight needle file kit.
Set the master cylinder, threaded end up on a large 3/4" drive socket, vice or some equivalent and give it a good smack with a heavy mallet.
Mine was pretty corroded so it required more force that I was really comfortable with, but it finally popped out.

The piston was pretty caked with scum but after cleaning, there were definite signs of water intrusion - Piston is scuffed and pitted...The bore was very difficult to photograph - (I couldn't find my bore scope!) but it's kind of a crappy looking surface and after seeing the damage to the piston, I'm glad I didn't order a seal kit - this one will get a new $150 master cyl.
Supports suspicion fluid was leaking past seals...
I really hope the caliper looks better but I won't get into that for a few days yet.
Keep ya guys posted if interested.