Cadillac Repair: brake hydralics, 1985 cadillac fleetwood, cadillac fleetwood

Question
1985 Cadillac Fleetwood FWD
I have replaced the master cylinder and the rear wheel cylinders.  I have bled the system as specified for a diagonal split system.  The problem is that the pedal rests low initially then fades to the floor. This is more evident when the engine is running.  I have at this point replaced the master with a rebuilt then a new one with the same result for both cycinders.
I am an ASE Master Tech and I am stumped.
I appreciate any advice.
Thank you,
Walter

Answer
Hi Walter, I'm not sure if I can give you any help here but I'll try. After replacing 2 master cylinders that should be eliminated as a possable cause even though it sure points to it eithor leaking from the back or internally. You probably checked the complete system for leaks already. Check the front calipers to make sure they slide freely. Have a helper push on the brake pedal and check the hoses for any ballooning and at the same time make sure that all wheels can't turn. The brake shoes should be adjusted just short of touching the drums. Another thing you could try is pinching the brake hoses off one by one with small teeth visegrips or pliers designed to pinch off hoses and see if you then get a solid pedal. At that point you will know what circuit is at fault and hopefully isolate it from there. The last thing to check is that the brake booster can hold vacuum and that there isn't a hole in the disphram. Take the valve off and pump it up with a mity vac and see if it will hold. It may have a small leak in it that the engine vacuum and pump can overcome with normal braking but sinks down with constant pedal pressure. Hope that helps. Bill