Hello,
I've got an R75/7 that I bought as a project bike. One of it's projects is no front brake.
I can bleed what looks to be the original M/C and it pumps up, works the caliper, and seems to hold pressure. However because there's a mess underneath the M/C of peeled paint, goo, and all that I suspect that the M/C leakes either internally, externally or both.
A new one is pricey to say the least at about $400. from MAX, kits are available for about $75..
But I'm wondering if I couldn't find a handlebar mounted brake/throttle/ switches from a later model such as an '80's R100 that could get rid of the undertank stocker and mount the M/C reservoir where I can watch it?
Good news and bad news....
► The diameter of the master cyl bore is written on the side of the unit. If you go the handlebar unit, then you'll want to go about 2mm smaller on the bore size. Yes, there is a preferred BMW unit that works with your existing BMW RH controls, but I don't have that info. A really smart person will be along to divulge that info.
► Most of the standard under-tank units can simply be dis-assembled, cleaned, and re-installed without having to have new parts. You'd need some really bad water damage to kill one of those units. The o-rings are all standard sizes and can be found at your local hardware store.
► Do Not forget to clean all the old brake fluid off the relays (left side), regulator (right side), and other electrics which are directly below the leaky m/c. The old fluid will appear as a thick mucus on the wiring and contacts. The corrosive effects of brake fluid may make parts of the electrical system intermittent by corroding the brass contacts inside the connector bodies and relays. And since the main power trunk has connections there, assume there will be issues sooner rather than later. Especially late 70's to early 80's models.
► While fixing the old m/c or moving a new m/c to the handlebar... this is an excellent time to consider adding "stainless steel" brake lines and modern brake pads. The problem is that you can't buy the brake lines until you pick a m/c location. Search the forum here. We've discussed how to build economical "stainless steel" brake lines in several threads.
🙂
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Great! Thanks for your thorough reply, Wobbly.
It looked bad when I first took off the tank but after getting all the surrounding components either off or pushed away it really isn't so bad that Naval jelly couldn't convert the rust and let me brush or scrape it away. The tubes aren't damaged enough to weaken them so a careful repaint should have them looking near new.
It's all accessible now so I'll go through each piece and either get it cleaned up or replaced as needed.
Something I hadn't thought of until you mentioned water damage is that the undertank location couldn't be much better for protecting everything mounted there from being damaged by water AND it's got a nice close heat source to help dry out whatever water might find its way, as water so easily does (like it finds its way up the cuffs of my jacket or down the back of my neck no matter how well I think I've prevented it). Makes me rethink the idea of moving to a bar mounted M/C.
Some follow-on thoughts....
► You didn't say the exact year, but my /7 is a 1979. For several years either side of '79 the main electrical power supply trunk of the wiring harness has a break in it at the starter relay. That, and the fact that a BMW on the side stand will drip brake fluid directly onto the starter relay is what leads to the electrical issues discussed above.
You most definitely want to remove the "mini-cube" starter relay and inspect it for external and internal damage. If you have the older relay with the metal can, then you most definitely want a new relay. You can easily source one off Amazon for about $8, but first you need to study the schematic on the side of the relay to determine which one you want. There are several different relays with very similar plug arrangements. So you need to be sure which you have before buying.
Typical plastic body Mini-cube
► I don't want to start a fire storm here on the forum, but since the master cylinder is out of sight and dripping corrosive fluid directly onto required electrical connections, you may want to consider a swap over to a brake fluid called "DOT 5 Silicone". That will put an end to the corrosive nature of the fluid for both the nearby painted surfaces and the electrical connections. It will also end the need for mandatory annual inspection and maintenance.
I warn you now that people feel very strongly about this fluid; they either love it to death or strongly oppose it. Getting truthful and authoritative information on the product is VERY hard to do. You might start with a phone call to Bel-Ray customer service, which is what I did 15 years ago. In the end, I replaced the DOT 4 fluid in my dual caliper RT with DOT 5 Silicone and have not had any further trouble with master cylinder leakage, corrosive damage to any part of the bike, or loss of braking system function. I love this stuff.
There is obviously more to both these subjects. I'm simply "skimming the surface" of both topics in case either sounds like it might be of help or further interest.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
The bike is a '77 built in July, 1976. The tag on the steering downtube specifies "R75/7" and I'm sure that because it's so early in the /7 run there's plenty of pieces carried from /6.
It's got just 12,500 +/- miles and the general condition gives me to believe that it's the actual milage instead of 112,000 as would be if the odometer had rolled over.
From the look of the brake fluid flow I think you're right that the bike sat for years on it's side stand
Old metal starter relay as cool looking as it is will be replaced. It's terminals were green at their bases and the whole thing is grungy and soaked. The voltage regulator seems to have escaped most of the brake fluid but I've got a new one coming anyway.
If I were to guess I'd say this bike was kept in a garage and covered for a very long time. If the master cylinder hadn't leaked.....well, it probably wouldn't take a lot to have it in shape to turn back the clock for me on these nice mountain roads where I live.
• I'm no BMW expert... that model could have either the oblong starter relay in the steel can. Those /6 type are pretty well protected from the corrosion. But, from your description it sounds like you have the 3/4" inch "mini-cube" type relay and relay socket that really get destroyed. You can get a new relay off Amazon for about $7, but be sure and study the schematic on the side of the old relay before ordering. There are several that look alike.
• One thing I find in common with all these leaking master cylinders.... the DOT 4 fluid has encrusted around the open end. This makes it impossible for the piston to fully retract to the circlip/ stop. On this type m/c, the piston must be allowed to fully stop against the circlip or the air isn't allowed to escape from the system while bleeding. You must have enough free-play in the op cable to allow that piston to come completely to the rear of its stroke.
Best of luck ! 🙂
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Apologies. The photo I previously showed for my new replacement plastic body mini-cube relay was not correct. Here is the correct relay for early /7 RT and RS bikes...
Notice the difference in the 2 schematics. The first has terminals 87 and 87a, so that power is switched between one terminal or the other. The relay needed for 1979 /7 has terminals 87 and 87. That is to say, 2 terminals constantly connected. These are a little more difficult to find, however a search entry in Amazon or other shopping network for the term "dual 87 relay" will turn these up quite fast.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Right.
As it happens I had one 'dual output' or dual 87 relay in my stash. Don't remember where it came from but out of nine relays that I'd put into a parts bin it was the only of the type.
I ordered a couple more and two relay sockets as well. I've sprayed the bike's socket all up and cleaned it best I could but a flishlight shows that it's still pretty foul in them little holes. I've never been lucky about getting terminals that are locked into a plastic socket out so I'm just going to cut out the old and put in new with new terminals, relay, the works. (It is kind of a shame that I couldn't find a steel encased Bosch relay but I never claimed or intended to be after a restoration..:) ).
The old relay seems to still work. With it's prongs all shiny I hooked it up on the bench and it does kick in and out like it should. Even so this bike lacks a kickstarter so it gets nice new hidden parts in the hope it'll keep working. I'm too old to be pushing my bikes down the road, or up hills to get a place where I can bump start.
The original part # 332 014 118 has apparently been succeeded by a part # 332 019 150. I hope so anyway, and the schematics do match.
► Before you go cutting up your harness, clean the relay socket in place with some contact cleaner, then apply a dab of No-Ox electrical contact protector compound to all the new relay terminals before you plug it in. That should cure all your electrical issues and save you a lot of time.
If you trace it out, power comes up to the relay socket from the starter motor on one of those #87 terminals. The other #87 terminal takes that same electrical power on a Red wire up to the distribution board inside the headlight AND the Red wire on the ignition switch. So the starter relay when wired in that configuration ends up being a vital part of the main electrical harness ! Meaning, if the relay is not plugged in, then there is NO power to anything else on the bike !!
► If you find it necessary to start cutting on your harness, then you should take it further with some corrective surgery. Those 2 wires formerly connected to both #87 terminals should be permanently joined (and duly insulated) so that power flows directly to the distribution panel and ignition switch irregardless of the relay. Then a single Red pigtail can be brought back from the distribution board to power the relay using either #87 terminal. That will add a great deal of reliability, and is in fact the way the starter relay was wired on later models.
► Before you get too misty-eyed about the metal can Bosch relay, pop the steel cap off and look inside. :ohmy: I'm glad they went to plastic can which seems to be sealed better and not nearly as sensitive to brake fluid.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Heh!.... I'm not going to hack and hew the wiring, just use a new terminal block with new terminals within.
It only amounts to cutting off the old corroded terminals inside tthe original block and using new terminals locked into a replacement block. It'll look very much like the original but will lack the 40 year patina coupled with the long exposure to brake fluid. Good connections will be enjoyed by all.
Yours is a good idea to bypass the relay except for that current to be applied during starting to the start solenoid. That way if the relay is toasted the bike will still run via a bump start. All that's lost is pushbutton starting with the starter. I'll suss the wiring with an eye toward doing just that, thanks.
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