Power Brake Booster Check Valve?

[steveb2001ZR2]

My brake pedal started going to the floor,no leaks new brake pads a few mouths ago.
I replaced the master cyclinder with a new one not rebiult.
I bench bleed it, bleed from farest the the closest 2 times.
adjusted back brakes .
It still goes way down,almost to the froor.I can push the brakes to the floor in drive and the truck will move , the brakes will not hold the truck back.
could it be the check valve ??
I dont hear it hissing, or anything.
when I press down on brakes it will have a very slight change in engine idle, not much.

[jdmccright]

What is your year, make, engine, etc? Are you getting a brake warning light on your instrument panel? Does it take alot of pedal pressure to get it to bottom out?

The check/combination valves could be faulty or clogged with debris, causing them to not close & seal properly.

Steady pressure on the pedal that causes the it to slowly sink to the floor indicates a system leak.

Also, if the system hasn't been flushed in a while, do so. Brake fluid is hygroscopic, absorbing water over time. You can bleed it many times, but if it is still weak or spongy, then the right procedure isn't being followed or there is air in the ABS system.

Autozone has a check procedure for the brake booster:
Vacuum Leak Test

1. Operate the engine at idle without touching the brake pedal for at least one minute.
2. Turn off the engine and wait one minute.
3. Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. If vacuum is present in the system, light application will produce less and less pedal travel. If there is no vacuum, air is leaking into the system.

System Operation Test

1. With the engine OFF , pump the brake pedal until the supply vacuum is entirely gone.
2. Put light, steady pressure on the brake pedal.
3. Start the engine and let it idle. If the system is operating correctly, the brake pedal should fall toward the floor if the constant pressure is maintained.

Good luck!

[steveb2001ZR2]

1989 C1500/chevy 350/auto/not getting any lights,it started going to the floor sitting at a light one day, I looked for leaks, found none.I replace master cyclinder.no more going to the floor, as far as when sitting at a light or just sitting with foot on brake.
But it seens to go down very far to me, like if it has very thin pads and very thin rotors and drums, which it dont. rotors and drums are good and thick, good and thick pads.
Could just be me, I have been driving a 2001 Blazer for the last 5 years, with 4 wheel dics brakes. maybe iam just not used to the old school brakes.
I just dont think there working as good as they should, Like I said, when I put the brakes on and stand on them, and give it gas, it will let the truck move, when I was a kid , thats how we burned the tires off dads truck, ( sorry Dad),it wont break the tires loose unless I goose it then it will.I would think it should not let the truck move forward with me standing on the brakes hard?
I pull a 16 foot trailer with this with 3 quads on it, and need to stop , no traler brakes to help.
Yes I did flush all the old out and have all new prestone fluid.
My brother has same truck and he drove mine and said, yes yours isnt near what mine is, I drove his, his stops way better and has alot better pedal.
we swamped out check valve , no diff.
bleed bleeder @ ABS under master cyclinder.