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Brake problem

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Michael Oritt

Michael Oritt

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Yesterday while returning from a ride in the 100 I applied the brakes and the pedal went to the floor--fortunately the emergency brake slowed me down enough that I could negotiate the upcoming turn and come to a stop.

After a few deep breaths I stepped on the brake pedal--this time it was normal and the brakes functioned seemingly fine. I checked the hydraulic fluid reservoir and it was full. I was less than a mile from the garage so returned slowly though the brakes continued to work fine and got me back.

In the garage I applied the brakes a few more times, all normal except for one time when I held my foot on the pedal it and it went slowly to the floor like I was pushing fluid out somewhere through a very small leak but there were no leaks. Next push the brake pedal was high and firm. Retested several times at 15 to 30 minute intervals--no problems, but obviously something is wrong here!

I haven't pulled the master cylinder off and apart yet but am thinking that the return valve is somehow hanging up. FWIW I have a BT7 discs in front and drums in back and a single circuit setup with a valve in the rear brake line which serves as a poor man's proportional setup.

Thoughts?
 
Since you have no loss of brake fluid, it is likely the master cylinder is causing this. That umbrella seal/piston that pushes fluid into the brake lines may have deformed enough to not seal. Not obvious how it could do this intermittantly, but I have heard of that happening. I would certainly pull the master cylinder and replace all the rubber parts inside.

Another possibility is boiling brake fluid due to dragging brakes. This is more likely to happen on the front discs. The fluid boils, you lose braking, then it cools off anc condenses and you have brakes again. What sort of driving were you doing when this happened? If you just left the garage, probably not boiled fluid. But if you were driving hard and using brakes heavily, perhaps so.

Good luck, keep dodging those bullets!
 
Nope on the boiling--it happened just after I started driving and no hard braking was involved in any case. I am sure I will find a bad seal when I get into the MC but because of the intermittent nature of the problem I was simply wondering if anyone else had experienced such a failure.
 
One possible cause...

I remember this as more of a soft braking action which has happened to me, but on a more than once occurrence.

I could not find any obvious signs of leakage anywhere and this is after I virtually rebuilt all of my brakes and whatever else was needed. But then, I asked a friend to pump the brake and I then noticed a subtle sign of wetness emanating from the infeed line to the master cylinder. Wasn't enough to pour fluid out over the side but visual enough to see that the connection wasn't air tight.

The threaded connection was already tight but after tightening more, I had no more problem with the brakes (after bleeding again, of course).

So maybe you have one connection that may be oozing just enough fluid out or allow air in, but not so noticeable. Just a thought.

Because of what happened to you, I realize it could still happen to me, so I do a periodic practice test on how fast I can grab and pull up on that hand brake. I also make sure nothing is blocking quick access to grabbing that hand brake in an emergency.

Good luck.

Paul
 
:iagree:
Yes based on my experience this is an initial phase of a master cylinder failure.They do not all give you a nice warning like this though.-Fwiw-Keoke
 
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