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Healey 100 Master Cylinder Failures

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M

mrunci

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Friends:
I have replaced three master cylinders over the course of the last year in my BN2. In each instance the symptoms are identical: even under light use, free play in the pedal progressively diminishes until the brakes effectively seize as the pedal remains glued to the floor. When the brakes are allowed to cool, normal free play of the pedal returns and for a time the brakes will operate normally. Each is a new Lucas/TRW unit which I chose since he car has been restored to AH concours standards.
Now I have read elsewhere of problems over the last several years with MGA units of the same manufacture, which reportedly have return springs that are overbuilt and do not release brake fluid as required because they maintain relief pressure too high. I presume this may be the problem with my units as well. I have also read of problems with units of other manufacture which are widely available on which there are reports of internal rubber component failure.
Oddly I haven't read of similar reports on the performance of either of these units in Healey BN2's - I may have just missed it since I've spent so much time parked on the side of the road.
Can anyone shed light on this problem, and ideally, on a solution. Periodic roadside stops to allow the release of brake fluid pressure could be enjoyable on a scenic country route, but are hardly practical on today's roads with lots of impatient fellow drivers.
Matt Runci
 
Matt:

My BN1 had this problem years ago. It was caused by the actuating rod adjusted too long so that the piston did not return to fully released position in the back of the master cylinder. This prevents the small port opening that allows brake fluid to bleed back into the reservoir when you release the brakes -- then the system slowly pumps up the brakes with each application until the shoes drag.

My problem was solved when I merely shortened the the rod slightly using the adjuster on it. Suggest you check that out -- If that doesn't fix it, make sure there is a clear open return path from the brake line to the reservoir when the pedal is released. You can test this by removing one brake drum and applying the pedal carefully to not let the wheel pistons get out of the cylinders. When you release the brake, the shoes should slowly return completely. If they don't, the return path is blocked. This condition can also be caused by improper fit of rebuild components in the master cylinder. This is doubtful since you have had the same problem with several master cylinders. I suspect the brake rod is the problem.

Good luck!
 
I may have had this problem with my BN2 a few years ago. But only the right front dragged! I replaced everything I could in the front braking system, including the MC. I was sure it was bad brake lining material. So I went over to disc brakes! No problem since. May not have been what you're experiencing but sounds related.

Oh, and this happened after driving the car for about ten years after it had been restored, including everything in the brake system.
 
Thanks Bill and Bill.
With weather in this area scheduled to improve this week I hope to check this out further. I know that adjustments to the actuating rod were a problem in an earlier episode but I thought that problem had been addressed with proper adjustments. I hope it is that issue again rather than the MC itself.
How much pedal movement do you have in your cars?
Thanks again.
Matt
 
It's hard to say exactly how much pedal play I have. It's neither excessive nor obviously too little. (Don't forget I now have discs in front which could make my perception different). I expect to drive the car a bit tomorrow so I'll see if I can improve on my perceptions. I don't think it's something that can be measured.
 
Mine has original drum brakes with about an inch of free play at the pedal. Beyond the free play, the pedal is rather firm, feels like it hits a brick wall. I do set the linings rather tight at the wheels, so there is a little drag when I turn each wheel by hand.

As per my post, you can test the opening of the brake fluid return path by applying the brakes partially with one drum removed. Let the pedal snap back and the linings and wheel cylinder should slowly retract fully pushed by the brake return springs. If they don't retract, your master cylinder is either not fully retracted or the bleed ports in the m/c are plugged.
The upward pointed front wheel cylinders on the 100's are also prone to corrosion. If the boots are bad, water can collect and migrate into the bores. This can lead to sticking brakes too, but a sticking wheel cylinder should not cause the pumping up problem you describe. So I think your problem is at the m/c, probably the actuating rod set too long. Make sure the return spring on the pedal is working also. There should be nothing to prevent the m/c from bottoming out against the "off" position.

Good luck, let us know how it turns out..
 
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