Classic/Antique Car Repair: MGB Brake system, caliper pistons, caliper piston

Question
I have a 1979 MGB. When I got the car the brakes did not work. I replaced the master cylinder, front calipers,rotors, rear wheel cylinders and shoes.  I cannot seems to get all of the air out of the system.  I have bleed the brakes a number of times and can see no air bubbles.  Could there be something else wrong besides air in the system? The pedal goes all the way to the floor.  It will pump up but goes down with foot presure.  I have checked the lines etc for leaks. Any help you can give me will be greatly appreciated.  Thanks

Answer
Hi LD,
If you can pump up pedal pressure and the pedal slowly goes to the floor and you do this several times and there is no loss of fluid in the reservoir then the fluid is bypassing a piston or both pistons in the master cylinder and returning to the reservoir.

If the pedal never builds up so that you can't force it to the floor then you either have air in the system yet or you have caliper piston movement or rear wheel cylinder piston movement.

You can test this several ways. One is to have someone look very closely at the rotor relation to the caliper on the front and pull up the hand brake while you pump the pedal. Look with a strong light as the movement can be small.

Another method is to get dead plugs to fit in place of the lines in the master cylinder. You install a dead plug in all ports at first and bleed at each dead plug to get all of the air out and you should have a rock hard pedal, then apply light pressure to the pedal. If the pedal goes down any at all holding light pressure for several minutes then the master is bad.

If it is OK install only one line at a time and note how much pedal is lost. Some will be lost as there is some movement of the caliper pistons. If there is a lot of loss of pedal, look at that caliper and rotor relation with a strong light while someone pumps the pedal. If one piston is stuck in the caliper the other one will push the rotor over to meet the stuck one thus the movement of rotor to caliper.

If the rear brakes are badly out of adjustment or the shoes don't perfectly fit the drum you will get too much movement in the rear pistons. You can somewhat check this by installing the rear line only and pump the pedal with the hand brake down then pull it up and pump again to see if there is any difference. If there is a lot of difference the problem is in the rear brakes. Adjust them again and / or remove the drums to see what is wrong.

With this isolation method of testing you WILL find the fault and thus be able to correct it.
Let me know.
Howard