Front Brakes, near Lock-up

[Schnell]

1981 R100RS with 82,000 miles.

I had braided lines installed 2 years ago and have admittedly not ridden more that 3,000 miles since. Yesterday coming up the driveway I noticed something was wrong. I stopped outside of the garage and then went to rolled it in using human power. As I was rolling it in I noticed it required more force than in the past. Put the bike up on the center stand and learned the front wheel would not spin. Today I began opening the front brake reservoir to have a look see and while doing so released pressure from it. I removed the brake fluid and reservoir to clean out the two ports. All was fine. I reassembled it and bled the system, or tried to. Got all of the air out from the calibers, but noticed bubbles coming up from the reservoir when ever I released the brake lever during pumping. In other words every back stroke bubbles rose from somewhere (one of two ports?). I put the cover back on and started pumping the front brake lever. I reproduced the same condition. Front wheel would not spin.

So whats up?

Ken

[ME 109]

Master cylinder cover off, drain fluid.
Observe very small hole in bottom of reservoir. This is a return hole, which will be blocked.
Poke fine wire in hole and clear.
No problemo.

[ME 109]

I'll add that it may be necessary to remove the plastic reservoir from the mc body to reveal the hole.
From memory there are two holes in the mc body. One supply hole and one return hole.

[Deleted User 62]

Schnell, when you cleaned the two holes in the bottom of the reservoir, did you remove the piston and clean the bore too? Pushing the blockage down into the bore, or using too hard a tool to do it and also pumping the lever while you are poking around in there, can all do damage to the seal between the piston and the bore. What did you use to clean the holes?

[Schnell]

First off, thank you for the reply guy's. I've gone through this once before and it was the case of the blocked tiny hole. So I was all set to clean it out again. What surprised me was the release of pressure this time. But I went along with the whole process anyway. I found the hole open (move the lever and air was forced up through the hole....hummm?) but put my tiny tiny drill in it anyway very gingerly. As suspected all was clear. So drained the fluid out and replaced it....well you know the rest. All said and done I've made no change other that thoroughly flushing out and replacing the brake fluid. Oh ya, the new fluid is a bit cloudy and I did see one small black particle in it befor I put the cover back on. Deteriorated rubber bits?

[Major Softie]

Oh ya, the new fluid is a bit cloudy and I did see one small black particle in it befor I put the cover back on. Deteriorated rubber bits?

Certainly possible. Are all your lines either steel or the new braided ones? If so, the only place you would expect to see such particles from would be seals. That can be in your master cylinder, but the calipers are where such things usually show up first.

Have you ever rebuilt your calipers? I certainly never expect a 33 year-old bike to survive with its calipers never being touched. If it had the fluid changed religiously, perhaps, but probably not even then.

When I buy a used bike more than 20 years old, I consider a caliper rebuild as necessary a part of getting it back on the road as an oil change.

[Schnell]

Softie:

The original brake lines were swapped out for braided SS versions about 2 years ago. I've owned the bike since 1991 and put the last 60,000 miles on the clock without a caliper rebuild. I'm not sure how that plays into the build up of pressure in the reservoir, but sounds like it should be on the repair list when solving this problem.

Anyway, I'm taking away from this post that a master cylinder and caliper rebuild are in my future.

Anyone else have ideas???

[ME 109]

Try removing one caliper and see if it's stuck tight. If both calipers are stuck tight, I would look elsewhere for the problem.

[Schnell]

In an effort to focus this discussion, my two questions are .....

1. Why is my front brake master cylinder reservoir being pressurized?
2. How can this problem be resolved?

[Ken in Oklahoma]

In an effort to focus this discussion, my two questions are .....

1. Why is my front brake master cylinder reservoir being pressurized?
2. How can this problem be resolved?

Not to be pedantic, but your master cylinder reservoir isn’t being pressurized. Your brake line and calipers, however, will be pressurized.

There are different possibilities as to the culprit. Therefore you’re not going to be able to rationalize a single fix. You’re going to have to do some things to eliminate certain possibilities.

You’ve gotten some good advice here and failing a plan of your own I would give some of them a shot.

For example, one possibility is that when you squeeze the lever the brake line(s) is pressurized, but when you release the lever the line pressure at the calipers doesn’t release. You want to know if the line pressure is returning to zero via the brake fluid returning to the reservoir. You’ve been told that a small orifice between the brake lines and the reservoir may be plugged and so the line pressure isn’t going to zero at the calipers. Now you can release the line pressure manually and see what you’ve got. You can do this by turning the bleed screw on the calipers out and a small amount of brake fluid will run out of it the pressure on the caliper pistons.

If the pressure is not released then the likely culprits include the afore mentioned orifice or a physical obstruction preventing fluid from moving back to the master cylinder reservoir. A physical obstruction could include deteriorated (on the inside) brake lines or some foreign object obstructing the flow.

If the line pressure is released, but the brakes still don’t release completely then you may have a stuck piston in a caliper.

I think your best approach will be to try some of the things already suggested and get back to us with the results. The logic of failure diagrams or “trees” can get a bit complicated when considering all possible failure modes.

I suggest that you try releasing the line pressure and let us know what you found.



Ken