I have replaced the brake booster. I have replaced the master cylinder twice and have gone through and bled the whole brake system at least 4 times. The pedal still goes to floor. I have tried everything I could think of.
With disc brakes there is no pressure retained in the system as there is with drum brakes
where there is a combination of discs and drums the discs have no pressure in those lines while the master cylinder has a check valve for the drums
when the brake pedal is depressed the primary cup immediately closes of the reservoir and the captured fluid is pressed to the caliper pistons and so activates the disc brakes
the first piston in the master cylinder activates the second piston in the master cylinder and again the primary cup captures the fluid the fluid and so moves the fluid through the check valve and the pressurized fluid overcomes the return springs for the shoes and brings the shoes into contact with the drums
when the brake pedal is released there is no return springs for the discs and so the caliper pistons only move enough to release the discs where the returns pull the shoes off the drums and returns the fluid by lifting the check valve off it's seat against spring pressure in the master cylinder
when this spring tension and the shoe spring tension is equal the valve closes off and there by pressure is retained in the drum lines
. Now it you have all discs it is the pedal movement that is not closing off the fluid hole at the primary cup or the compensating hole is not being covered with the piston movement
If you have bled the system correctly using 2 persons and starting at the longest line first then I have to agree with Dan's statement that the problem is in the master cylinder especially if the replacement master cylinder was not out of the box new
some GMC product master cylinders require that the brake switch in the middle of the master cylinder has to be removed before a bleeding process as the plunger stops the second piston from activating
Seek advice from a specialist brake shop as it is difficult to diagnose properly from what is written
If you are loosing prime in the system but not loosing fluid my best guess would be the master cylinder.
Testimonial: "I've changed the master cylinder twice and went through the same process and still am having the same problem"
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Hi James Kwaak III, I want to help you with your question, but I need more information from you. what do you mean by hold? Can you please add details in the comment box?
Hi Brad, I greatly apriciate it. When I press the pedal its soft until the second or third pump. let it sit for about 10 seconds and it loses pressure. I have primed the master cylinder and bled each wheel cylinder at least 6 times
I also replaced the brake booster as well and still have the same issue
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