1978 R100/7 Front Brake Caliper Sticking/ Won't Disengage. Suspect: Master Cylinder?

Restoring my Dad's old 1978 R100/7. Sat garaged for over 20 years. Front brake caliper seized in storage. I drained the oil (2 years ago) out of the front caliper in order to transport the bike.


Actively working on the bike now.


Rebuilt the front caliper. Bought a new brake line. Rebuilt the master cylinder. Bled the brakes, and still have the same problem I started with: front brake caliper is seized/ won't release. After some sleuthing with compressed air, I believe the culprit is the following master cylinder jet:


Bought a guitar string (high-E) to run through this passageway next week and see if that helps. Wanted to check here first to make sure that fluid (compressed air test) is SUPPOSED to flow that direction.

Other info that supports my suspicion of this pathway: filled the reservoir with brake fluid, repeatedly pumped the handbrake (hard) to push fluid down the line. Fluid eventually arrived, so I know the pathway is allowing flow that direction (from reservoir to caliper). Next, tried the reverse, pushing fluid with a syringe and tubing from the caliper to the reservoir: nothing doing. Friend pushed so hard the tube popped off the plunger and sprayed brake fluid everywhere. That's what led us to investigating with the compressed air, and eventually, suspecting this one pathway.

Thoughts? Thanks everyone.

Nice bike! It does sound like the return hole in the master cylinder is blocked. A few other things that don't allow the pads to release are 1) old, flexible brake hose; 2) o-ring around piston in caliper not functioning properly or 3) the plunger at the aft end not being properly adjusted. On this last one, if it's not adjusted correctly, the O-rings on the internal piston of the master cylinder won't be in the right position as it moves back and forth...if not in the right position, then fluid can't get past it on the return. There might be others, but that strikes me as the likely culprit. But you do need to see fluid come back into the master cylinder. Often times you have to be careful you don't get a geyser there when pumping the brakes during bleeding.

Have you taken the master cylinder apart yet? Post pictures.

Found this on the web. Good pictures if you get lost in the direction of the seals and spacers.

http://s428.photobucket.com/user/gruntyman66/library/78 R100 14mm Front MC Rebuild?sort=2&page=1

evojet said:

Found this on the web. Good pictures if you get lost in the direction of the seals and spacers.

http://s428.photobucket.com/user/gruntyman66/library/78 R100 14mm Front MC Rebuild?sort=2&page=1

Click to expand...

That link is in this thread - Rebuild-Sites-with-Photos. Probably not well advertised, but there are some good visual threads there.

If the brakes still drag badly after you finish messing with the master cylinder, you need to replace the pads. VERY common to get excessive brake drag from old pads.

AntonLargiader said:

If the brakes still drag badly after you finish messing with the master cylinder, you need to replace the pads. VERY common to get excessive brake drag from old pads.

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Thanks. I'll get new pads then. Any recommendations on brand or best place to purchase?

20774 said:

That link is in this thread - Rebuild-Sites-with-Photos. Probably not well advertised, but there are some good visual threads there.

Click to expand...

Thanks everyone, super helpful.

20774 said:

Nice bike! It does sound like the return hole in the master cylinder is blocked. A few other things that don't allow the pads to release are 1) old, flexible brake hose; 2) o-ring around piston in caliper not functioning properly or 3) the plunger at the aft end not being properly adjusted. On this last one, if it's not adjusted correctly, the O-rings on the internal piston of the master cylinder won't be in the right position as it moves back and forth...if not in the right position, then fluid can't get past it on the return. There might be others, but that strikes me as the likely culprit. But you do need to see fluid come back into the master cylinder. Often times you have to be careful you don't get a geyser there when pumping the brakes during bleeding.

Click to expand...

Excellent feedback, thank you! I'll report my findings.

when was the last time you changed your fluid ? Might be moisture in the lines. Might need to disassemble your front calipher. They do seize up from non use, sitting outside, moisture in the air, etc.

anerker said:

when was the last time you changed your fluid ? Might be moisture in the lines. Might need to disassemble your front calipher. They do seize up from non use, sitting outside, moisture in the air, etc.

Click to expand...

Welcome to the forum!
Gary

Did you fix your brake issue?

Micro bubbles in master cylinder reservoir while bleeding

Hi All,

It's been a long winter!

I've put in a steak less steel brake line, new brake pads, and rebuilt the master cylinder again (using all the kit parts this time [last time I elected to use the old parts that looked in good condition- stupid]). So now I'm bleeding the brakes, and I've squeezed the brake lever about 10,000 times, and all over are these TINY micro bubbles floating up. No added pressure in the lever squeeze, no fluid out the bleed port on the caliper. Any thoughts?

I'm sure you've bled brakes before...pump the handle, hold the handle, crack the bleed valve, close the bleed valve, resume pumping the handle and repeat. I just did the brake on my /7 and had no real problems, but all I did was feed new fluid into the reservoir and run it through bleed valve. I made sure that I had a piece of wood between the lever and my handgrip to keep the internal piston from moving too far...often times, just outside the normal reach of the piston corrosion can set up which will tear the o-ring. Given that you redid everything, that's not really your problem.

You might have to undo the master cylinder from being clamped onto the frame backbone and reorient it in order to help with getting the air out. Normally it sits with the outlet on the upside. I think you should tilt the back up so that any air can migrate backwards.

Another thing that you can try are the systems that push brake fluid from the caliper up to the master cylinder.

Hopefully others will chime in with their experiences.

1) You should loosen the clamp and tip the MC slightly nose-down for bleeding. This is a fundamental thing for this system.

2) You need to set the cable length so that the piston retracts to the right position. Your toolkit has a little fork in it that is the setting tool for this. You adjust the cable at the MC until the tool just slips into place. After that there is no more cable adjustment that you make anywhere. There is no adjustment at the hand lever.

I have seen that the line that is flexible (rubber one) that connects from main frame to turning fork will get so clogged over time (as rubber breaks down inside) that it will allow some flow for braking (under tremendous pressure) but not allow backflow (not under so much pressure).

Make sure this hose is fully clear of any debris.