Brake Pedal problem on a 97' gmc jimmy?

[werty]

I have a 97 gmc jimmy. The brake booster went out on it. So I replaced. in doing this I followed all procedures...the rod length is the same as the old one. Because I didn't even open up the hydraulic system I shouldn't have to bleed the brakes but they were going to the floor now...So decide to install a new master cylinder....I bench bled this and followed all diagnostic instructions to a t.. The problem that I am Having is that when the car is off it has brakes but when you turn it on the pedal goes to the floor. all the brakes are good. and it has rear drums.( the wheel cylinders are good and the shoes are adjusted out) any ideas what could cause this problem?I am going to also add the car stops fine but it is as if there is no reserve pedal it goes to the floor.....it has abs as well and i was told you have to use a scan tool to bleed this if you suspect air of being trapped inside the unit
I have replaced this with a new one a still the same problem........

 

[August]

the vacuum check valve is not working (the thing that connects the booster to the vacuum hose)
TEST for faulty check valve
remove the check valve , you should be able to suck in it but not blow in it (hose side) blow out but not suck in (booster side).
TEST for faulty booster.
suck on the vacuum line going to the booster you should be able to make your tongue stick to the end of the hose
P.S. you dont need to use the scan tool unless the ABS light is staying on. (it's normal for it to come on for a few seconds when you start the engine)
you will not be able to bleed the brakes if the if the ABS light is staying on.