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MG MGB Technical - Brakes sticking

I have a 1980 MGB with some brake problems. The front brakes were hanging up so I replaced the flex lines and the calipers bled the brakes and everything looked good in my shop. I test drove the car and the front brakes were stuck again. I have checked the brake switch, it is ok. When they are stuck I can crack the bleeder and a small amount of fluid comes out and the brakes release. I think I am down to the master but because they are expensive I wanted to see if I missed anything. Its funny I can work the brakes in the shop many times and they function OK. I can even start the car( using the booster) and they work. Its when I drive it they fail. Any advise would be of great help.
Thank you,
B. A. M. Bruce McCormack

Do you have a servo fitted?
Dave O'Neill 2


Over here, a booster is another term for a servo. I can see how you might have read that as a battery booster. An 80 would have an integrated servo rather than a remote.

C R Huff

I once bought a car that every time I stepped on the brakes it would lock up. bleeding the brakes solved the problem only until I stepped on the pedal again. I looked and looked. Finally found a partially crushed brake line behind a clamp.


Yes it has the servo on it. I have read even more on brakes and am tending to thing the return hole in the master is blocked by something. Today I am going to take it for a spin using only the hand brake. If the story I read holds true when the brakes heat up a bit and the fluid expands the brakes will get tight on their own as the return hole in the master could be blocked not letting the expanding fluid return to the tank on top of the master. I will post my results. Thank you so far.
B. A. M. Bruce McCormack

Did you do the 'crack the bleeder' test first? If not you should have! If both calipers are binding on and opening either bleeder releases both, then the problem is at the master. Two problems at least can cause this - the master piston not coming back far enough to clear the bypass holes to the reservoir, or possibly the breather system in the cap being blocked. If it is this last then simply unscrewing the cap should release the brakes. If that doesn't do it, but opening a bleeder does, then it is the piston. I have read of one account recently where the push-rod between the servo and the master piston was too long, so that even though the pedal was coming back all the way i.e. it wasn't the brake light switch causing the problem, the piston was still partially operated.
PaulH Solihull

I had a similar problem and the solution that worked was unscrewing the brake failure switch from the bottom of the MC (not all the way) prior to bleeding and once bled, then screwing it back in. Another possibility (according to the shop manual) is that the free play in the brake pedal is not adjusted properly.
B Dubuc

This thread was discussed between 11/08/2010 and 12/08/2010

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