been boosted for like 3 days now but for some reason i cant brake every time i brake on my car it get really stiiff and resist on the pedal and i cant stop i need help soon and yes i do ahve a check valve on the brake booster it came stock wtih it . so we took off the brake booster line and test the car again and it still had the same problem ? help please i cant stop ! also when i get to a lower speed the brakes starts to act normal ? wierd?

 

the brake booster needs vacuum. the vacuum hose from the brake booster is probably not hooked up to the intake manifold. check it.