QuestionQUESTION: Hello, I asked a question on brakes that lock up on a 70 Midget. Howard gave some suggestions which we have followed, with the following results. The master cylinder is new. After driving the vehicle for about a half mile or so, the front brakes lock up. When I loosen the bleeder screw, on either of the front wheels, the brakes unlock and can be driven once again. The pressure is building in the lines and not releasing. Any other suggestions? Could it be a bad master cylinder?
Thanks
ANSWER: Hi Dave
If the pressure is building up but not releasing, then this is usually a collapsed flexible brake hose which is allowing fluid through under pressure, but not allowing it to return when the pedal is released. This would only affect one front brake, not both of them.
It is easy to check. If the master cylinder is faulty, then all 4 brakes will lock up, including the rear drums.
Are you using DOT5 silicone brake fluid? I have had several older MGs with this symptom, caused by DOT5 fluid. The main rubber cup inside the master cylinder swells up and covers the fluid return hole, causing the brakes to lock on when they are applied.
If you are using DOT5, get back to me and I will tell you how to fix the problem.
---------- FOLLOW-UP ----------
QUESTION: Hey, thanks for the reply. I mis-typed the year of the MG (sorry, I'm old!) It is a 79! The front brakes are the only ones locking up. When I loosen the bleeder valve on either front wheel, they unlock for a short distance before locking again. The car was purchased from another owner who put in the new master cylinder. It state "Dot#3" on the cap so I assume that is what he used. The brake lines also appear to be new.
Thanks,
Dave
AnswerThe 79 midget had a different master cylinder, with separate pipes for the front and rear brakes. If both front brakes are locking, I would suspect the master cylinder has a faulty cup inside it.
I would strip it and replace all the rubber seals.
ps - Use DOT4 for topping up.
