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MG MGB Technical - Strange problem with brakes

Hi

I've got an odd problem with the brakes on my 77 roadster. The front driver's side front brake sticks-on. When it does this the pedal is rock-hard.

It does it fairly infrequently - certainly not every time the brakes are applied - and it eases off in a matter of minutes and things are back to normal for a while.

I can't imagine what the problem is. If it was a sticky piston in the caliper it wouldn't make the pedal hard and it would stick every time it was applied. I think it must be master cylinder related to affect the pedal, but I can't see what could be wrong.

Any ideas?

Thanks

Nick
N Wilson

I would consider changing both front brake hoses. They can cause this happening. The inner hose closes up and the small pressure from the front calipers cannot force the brake fluid back into the M/C.
Sandy Sanders
Hudson Florida
conrad sanders

I agree with Conrad, your car is at the right age for brake hoses to fail and from the symptom you have described I would start by looking at left brake hose for "sweating" around both fittings. It's best to replace both front hoses and the single hose in back to avoid future brake problems
Steve

Thanks for your input. I'll have a look at the front flexible pipes - they are about 6 years old now (stainless replacements). I can't see it making the pedal hard though...

I see that the manual suggests that the "master cylinder vent" may be blocked. Is this an air vent in the reservoir lid, or something else?

Regards

Nick
N Wilson

..was driving a roadster down the A3 at the devil's elbow yesterday - familiar smell of very hot brakes - so stopped and found the front right disc glowing nicely - clouds of smoke - pedal quite hard (boiling fluid) - brake freed itself so I drove on but the brake was normally squidgy again.

Oh well - another caliper kit................ hope the bearing and the hose are still OK.

Roger
RMW

I've never heard of the stainless/teflon lines swelling and closing in a way that would cause your problem. I suggest checking that your brake pedal returns fully allowing the master cylinder to fully return. If the piston is partially depressed it can block the fluid return hole and cause the brakes to lock. Might even be the adjustement of the brake light switch, which if mis-adjusted can hold the pedal partially depressed. Another cause can be need of lubrication on the pedal shaft.
Bill Young

Ah. Two interesting responses here.

I've had the smell of hot brakes and clouds of smoke experience. So is it boiling fluid that causes the hard pedal? I'd have thought that brake fluid vapour would have resulted in a spongey pedal...

The other interesting point is that I just recently adjusted the brake light switch - as the light was a little too keen to stay on.

I'll investgate that further tomorrow.

I've had a closer look at the brakes now and I'm quite alarmed to see that the the plumbing is decidedly odd. The Haynes manual shows the primary piston in the MC operating the front brakes and the secondary piston operating the rears. My car has the front left (passenger) brake operated by the secondary piston and the rest operated by the primary...

Thanks

Nick

N Wilson

Nick; Haynes is right on the plumbing. You need to get your plumbing right and go from there. Good luck.

Clifton
Clifton Gordon

Nick
Good idea to get the plumbing right first.
FWIW, I had a similar problem on my '67 roadster.
symptoms; hard pedal,smelly brakes, occassional dragging brakes.
Thought for all the world it was rebuild caliper time but my mechanic traced it to a bad booster-pressurising the system combined with the master cyl. circlip had come out of its groove which mean't the only thing holding the master cyl. together was the pedal. Apparently some m.cyl. resleeves don't have a deep enough groove to retain the circlip.
So recon. booster, new m.cyl, new rear w'cyls and a few $ - hey presto no more problems.
Good hunting
Gordon
GORDON

This thread was discussed between 24/04/2005 and 26/04/2005

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