The power brakes on my 98 Jetta keep malfunctioning. The car can stop but I have to push the pedal hard get it to do that. I just had a new booster installed and have taken to the mechanic once after the installation. They tightened the hoses from the engine to the booster, but the brakes have gone bad again ( it has been two weeks)...any ideas as to what the problem could be?...Dennis