I feel like a NEWB asking this question - brake pedal pushes back when powerbraking

[8UWITH6]

Installed VAC brakes on the GN this spring. New Master Cylinder, USED Booster. Good VAC block with new hose and filter and check valve. Started the car up in the garage the other day to let it get up to operating temp, cycle powersteering, shift through gears, etc. Put the car in Drive, stuffed the brake pedal to the floor and began giving it throttle and the pedal would kick back at me each time..............

BOOSTER BAD? LMK, thanks guys.

 

[TurboBuRick]

Sounds like the check valve is not holing back the boost. Pull it off and check it out.

 

[8UWITH6]

Not going into boost........... just off idle bringing the vacuum down close to 5 or 0