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MG MGA - Brake problem

Yesterday I was approaching the highway turn off for home and put the anchors on, only to suddeny find my foot halfway to the floor, instead of the usual 1" or so down. Fortunately I had brakes but it was very unnerving.
I drove home carefully, testing all the way, pedal still low.
Investigation today revealed nothing aparent - fluid level OK, rubber hoses no hernias, free play at the m/c about 1/32". And to cap it all, the pedal travel seems almost normal.
Probably a red herring, but the RH disc chatters after about half hour's driving if applied softly, but OK if braking harder. The brakes start out each journey feeling wonderful.
The mc and calipers are relatively new.
Any thought on what I should look for next?

Art Pearse

I'd check the mechanicals first. Rear brake shoe adjustment and condition of adjusters; front disc runout; and wear on master cylinder clevis, clevis pin and brake pedal lever hole. It might also be good time to check and grease the brake pedal pivot bush and pin.
Mike Ellsmore

Hi Art
Two things--
First it could be a sticky master cylinder piston but usually they tend to play up first thing when driving and then by pumping the pedal a bit they come right till next time
Still a possible though--If it is the M/C that 1/32" freeplay would be much more while it's sticking, so when you have the problem if you check that clearance then it will tell you if it's the cylinder or not--If it's stuck down a bit there would be more clearance
I'd be tempted to jack the front wheels off the ground and check for excessive wheel bearing movement--pushing the pads back and by turning the wheel by hand see if you have a wobbly disc rotor -when turning the wheel you would feel or hear tight spots
That RH one could be the culprit
William Revit

I had a similar with the brakes on an Audi we had a number of years ago. I was going down a medium hill with a signal at the bottom - signal was red and I pushed the brake pedal, which fell to the floor - I did a real quick up and down on the pedal and almost put the car on its nose. It all happened so fast that I wasn't sure what had occurred - my wife asked "what was that all about"? I checked the fluid level and checked everything I could think of after we got home with no further incidents. For several days I was walking on eggs whenever we took the car out. It was a week or so, when the situation occurred again and I pulled the master cylinder apart and found a radial split in the main seal. Rebuilt the master cylinder and had no more problems. Cheers - Dave
DW DuBois

Mike, the brake pedal bush does need renewing - there is about 1/2" of lateral movement at the pedal and with a LHD car the pedal spacing is critical! So I intend to renew both pedal bushes.
Disc runout seems to be OK, based on the fact that the pads are in contact, just a slight drag. I checked the mounting bolts - all good. Wheel bearings good as are the tie rods.
Dave, your situation seems to be similar, so I think I will examine the mc parts.
Art Pearse

Art. What type is your fluid and when was it last changed. I once had a similar problem on a company Cortina and the problem had gone the next day. And never came back. I always thought I must have boiled water that had been picked up by the brake fluid despite the car being correctly serviced. I had driven down from a ski resort. Y the way. Probably not your issue but just in case.

Paul Dean

I recently had a similar problem with the clutch, and on dismantling the MC found that the piston seal had come out of its groove on the piston and was loose on the piston, so all the sealing was being done by the bus seal behind the piston. Setting it back in position solved the problem.

A friend had exactly the same issue with vanishing brakes, and it was also intermittent. I replaced the MC with an AP unit I had in stock and the problem was solved. Unfortunately I just threw the old unit in the trash, and can't say what the root cause was there.
Dominic Clancy

Paul, Dot4 and 3 years old.
Art Pearse

This thread was discussed between 05/08/2017 and 07/08/2017

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