Replaced a Master cylinder and the brakes are still not working right.

What does "not correctly" mean? Pedal goes to the floor? Rock hard and pulling to one side? Rear brakes are grabbing?

Caradiodoc

I'm guessing you are not seeing the fluid level go down from a leak.

If you can pump the pedal up rapidly, then hold it and it stays up, then after releasing it for a few seconds it will go to the floor again, suspect the rear shoes are out of adjustment. If the original problem had to do with the front brakes and parts in the hydraulic system were replaced, there could still be trapped air that gravity bleeding won't get out. Two ways to address that are pedal bleeding with a helper or use a flat blade screwdriver to pry each caliper piston back into the housings. That will push the fluid back up to the reservoir and any trapped air bubbles with it.

If you aren't already aware of it, never push the brake pedal all the way to the floor when bleeding the brakes because the lip seals can be ripped when they pass over the crud and corrosion that builds up in the lower halves of the cylinders where the pistons don't normally travel. The exception is when you install a new or freshly rebuilt master cylinder. Those don't have any corrosion yet.

Of course you understand that after replacing the front pads you have to pump the brake pedal part way to the floor numerous times to push to pistons back out before the pedal will get hard.

What other work was recently done to the brakes? Did the low pedal just appear on its own or after some other service? Some history would be helpful. Does the car have anti-lock brakes?

Caradiodoc

It sounds like there's a leak in the hydraulic system and it may be sucking in air when you release the pedal. Look closely at the steel lines that run under the left side of the car, and at the rear wheel cylinders for signs of wetness. If nothing is found there, look for fluid seeping from between the master cylinder and power booster.

Has the red brake warning light turned on? If so, one half of the hydraulic system is not building pressure.

These cars were also available with rear disc brakes, (I have one). If you installed a master cylinder for a car with rear disc, there might be a difference. I'll have to research that. Normally there is a residual check valve in the port for the rear brakes, but this is a split-diagonal system. One hydraulic circuit works one rear brake and the opposite front brake. There can't be a residual check valve holding pressure on the front brakes or they would stay applied.

You might try bleeding the rear wheel cylinders again. Watch for air bubbles. If there are some, there is likely corrosion buildup that is holding the lip seals from sealing after they have been applied. That would let fluid dribble out and air get in.

Have you had the rear brakes apart to inspect them? It's not unheard of for one of the linings to rust off and rotate with the drum. You will have good pedal or no pedal depending on where the lining is when you hit the pedal.

Caradiodoc