100 psi compressed air is the trick. Sometimes they will cock off at an angle and get stuck, so make sure the piston is coming out straight.

Heating the caliper may not do anything for you. The piston-caliper fit is VERY close. The DOT4 fluid attracts water which can make the piston develop surface rust. Trouble coming out is usually associated with this rust, to a degree that a new piston may be required.

A thorough washing of all parts in hot soapy water is highly advised, followed by a thorough heat gun drying.

Good luck.

@wobbly Thank you once again Richard. “Never Quit” is my new self imposed slogan. Additional persuasion with the air compressor, penetrating oil and threats from me finally removed the piston! Washing with hot water and soap followed up with compressed air drying is a good tip too. Thank you sir!

@wobbly Now that I have both pistons out I've discovered that both of them have one small spot each of pitting. Short of replacing both with two new ones is there an acceptable method of repairing the pitting? If so, what type of shop should I look for to make an inquiry about a repair(s)? 

Thank you in advance.

78 R100(RS) - I very recently repaired my 40mm ATE front brake calipers. Like yours, there was a fair bit of corrosion. I was able to clean up the caliper bodies nicely, and the bores were clean. The pistons were a bit of a mess. I felt, being original 44 year old parts, I would go ahead and replace the pistons. I bought the kits from our good friend Ted Porter at the BeemerShop, along with new brake pads. The new pistons are hard anodized alloy, rather than steel. All is working very well, now!

@4949 Thank you again David. I’m going to follow your lead.

Posted by: @bruce-harris-jr

@wobbly Now that I have both pistons out I've discovered that both of them have one small spot each of pitting. Short of replacing both with two new ones is there an acceptable method of repairing the pitting? If so, what type of shop should I look for to make an inquiry about a repair(s)? 

Thank you in advance.

Like David said.... You can do all this yourself. There's only 2 parts, plus seals, and you've already completed the hard part !! So you don't need to take this to a shop.

The next question is... is the corrosion on the wet or dry side of the seal ? If it's on the wet, then regular changing of the brake fluid, as BMW suggests, will keep new corrosion at bay. If the pitting is on the dry side, then of course the moisture around the seal will always be ready to combine with wash and rain water, bringing the corrosion right back. But that might take several years. 

If the bike is being sold, then I might install new pistons. If the bike is being kept under your service schedule and the pitting is tiny (smaller than 2mm), then regular brake fluid changes might be all that's needed. It's a judgement call.

Reassembly Notes

• Undoubtedly, you are seeing a brown varnish on some of the parts, and maybe even some white corrosion 'bloom'. All that needs to be washed out with the hot soapy water. Any soap, brush or scouring pad you use to wash dishes will be fine. In fact, a trip though the dish washer is OK too. You simply want to start reassembly with clean, dry parts.

• The seals should be soaked in brake fluid before fitting and the "wet side" of the piston and caliper need to be wetted before assembly. 
• This is an excellent time to change over to "stainless steel brake lines", if you have not done so. 
• If there was enough water in the caliper(s) to cause trouble, then it's also in the master cylinder too. That will need to be removed, disassembled, washed, re-sealed and reassembled just like the caliper. Use a 1/4" wooden dowel about 4" long as a cylinder hone by wrapping a 1" wide strip of 300-400 grit emery paper around one end and chucking the other end in an electric drill motor. It will be maybe 2-3 hours (total) of work, without much finesse, technique or technology involved. This is why this is best done by you and not a shop. The only thing a shop can do is add $400 labor cost, which you don't need.

Hope this helps.

@wobbly Yes indeed it does help Richard. I’ve already done the master cylinder overhaul and will be ordering new brake lines, caliper kit, speed bleeders, and a pair of EBC sintered brake pads when I return home from the MOA rally next week. I plan to keep this bike until I can no longer ride. It will then be given to one of my grandsons. 🙂 

Sounds like you're headed in the right direction, Bruce.

Just a couple of thoughts.

It's been a while since I took a caliper apart, but if I remember correctly, the seals fit into the caliper body, and in use don't move. If I've got that right, then the piston moves up and down on the seal. Seems to me if the piston is corroded, it's going to have a difficult time sealing on that rough surface. Glad you're replacing those pistons.

I've never had much luck polishing pits out of a master cylinder bore. Even if I get it shining bright, they always seem to leak. And last time I bought one, the reseal kit was something like seventy-five bux, money that went right out the window when I had to buy a new master cylinder to get going again.

Posted by: @8166

I've never had much luck polishing pits out of a master cylinder bore. 

• It makes a huge difference 1) where the pitting is, and 2) how deep the pitting is. But, good or bad, I believe it's impossible to gauge any of that until the cylinder is honed. 

• The pitting occurs where the water (brought in by the DOT4 brake fluid) is allowed to sit. Which is why you need to have a regular program of brake fluid bleeding. 

• An option is to get your brake master cylinder re-sleeved. That offers the possibility of going smaller, thereby increasing braking efficiency.

@wobbly The BMW parts fiche doesn't show any repair parts for the calipers on my BMW R100R.  Before I jump into the repair of my brakes I need to be sure there are spares I can fit after I split the calipers.  I assume that reuse of the present seals would be penny wise and pound foolish.

After replacing the pads on the front calipers they fail to retract when the brake lever is released, causing drag on the rotors.  I assume there is corrosion on the pistons, even though the bike overall was clean and corrosion-free when I bought it. I plan to clean the calipers and pistons as best I can with a small brush and some detergent before I split them.  Maybe I will be lucky enough to fix the problem that way.

BTW, the bike has 40K miles on the clock and appears to have the original pads, which measured just under 2mm. 

Posted by: @632

@wobbly The BMW parts fiche doesn't show any repair parts for the calipers on my BMW R100R.  Before I jump into the repair of my brakes I need to be sure there are spares I can fit after I split the calipers. 

And before anyone jumps in with repair help... we're going to need a year model.

I assume that reuse of the present seals would be penny wise and pound foolish.

Why would you assume that ? If the parts are good, use them.

After replacing the pads on the front calipers they fail to retract when the brake lever is released, causing drag on the rotors.  I assume there is corrosion on the pistons, even though the bike overall was clean and corrosion-free when I bought it.

And I think you've made a bad assumption. Read...

https://www.airheads.org/community/parts-is-parts/brake-hose-failure-causes-cures/#post-6217

This is why we need "Stickies" OR a "10 most wanted" maintenance sub-forum !!

I've reemerged from the witness protection program and sorry for the long delay in responding. First, thanks to each of you for your responses and suggestions. I appreciate it! Second, my wife and I are in the process of moving from our home of thirty-seven years to a rental condominium while we wait for the construction of our new home to be completed in about one year. Who cares Harris? Well, my lettle ole '77 R100S really does care as it's restoration project is on a temporary hold as my garage/shop has been dismantled and moved to a storage facility. So for now all I can do is read and gain additional information on our wonderful (sometimes) Airheads. Ripping and ridin' on my R1250GS will keep me entertained until then. 

On the sidelines and waiting to get back into the game ASAP! 

@wobbly I struggle to understand why replacing rubber seals that date from the early 1990 isn't a no-brainer.  Rubber ages.  It becomes less pliable and ultimately cracks and fails at it's job. Replacement is cheap insurance.  I replaced the brake hoses for the same reason.  I've long been a supporter of stainless steel hoses, despite what Oak said years ago.  It took manufacturers some  time to see the light, but today they often fit those right on the assembly line.  

BTW, all R100R models use the same front brake calipers, no matter the year.  The only difference I'm aware of is the early machines had but one caliper in front.  

Posted by: @632 

@wobbly I struggle to understand why replacing rubber seals that date from the early 1990 isn't a no-brainer.  Rubber ages.  It becomes less pliable and ultimately cracks and fails at it's job. Replacement is cheap insurance.  

And I agree with you. What I'm saying is that it's a judgement call. If you KNOW the seals have been in place for 20 years, then it doesn't hurt anything for them to be replaced. But if they've been in place for only 3, and show no signs of wear or abuse, then why replace them. If you swap a good part for a good part, exactly what is the expected benefit ??

BTW, all R100R models use the same front brake calipers, no matter the year.  The only difference I'm aware of is the early machines had but one caliper in front. 

IMHO, your knowledge is limited and your thinking is extremely selfish.

1) The whole idea of "community" is to share. That's "share" as in the 1984 ABC model under discussion most likely has the exact same brake system as the next reader's 1984 XYZ model. By adding the year model to the title, you help the NEXT GUY in his forum-wide search (2 years from now) for brake issues with his/her 1984 brake issues. Take off the blinders. It's not always about YOU.

2) Except for one R60 back in the mid-1970's, I have only owned RS models since 2001. I wouldn't know a CS from an R model. Fortunately, most of the differences are in cosmetic and final drive ratios, 2 items which rarely ever enter the discussion on this forum. This is why all the repair manuals base their notes on year model. 

And to be more specific, there were 2 caliper brands used on Airheads. To remove a piston from an early model ATE caliper, differs dramatically from the way you remove the pistons from the later model Brembo. So the year model must be known in order to understand which brand caliper is under discussion. 

3) I volunteer my time here to both Moderate and answer technical questions, a task I do on a daily basis. Most of my responses take well over 1 hour to research and write. I am no longer an ABC member; I am allowed to post here because of a mutual sense of community. (There's that word again.) While I'm happy to share my knowledge and unique perspective based on 30 years as a professional motorcycle mechanic and being a mechanical engineer, there is a limit. If an individual chooses to get a little testy about something as benign as a year model, then the pool of probable volunteer responses might also shrink accordingly. 

I hope your question is thoroughly answered.

All the best, my friend.