Every so often the power assist would go out on my brakes. Now it happens all the time. There is a little bit of assist when braking at higher speeds, but it completely goes away as the car slows down. No power assist at low speeds anymore (backing up, parking, etc.). The vacuum pump seems to be working (constant suction with the engine running). The hoses seem good though a bit old, and the check valves seem okay (I can blow air in one way but not the other). Every now and then the air duct vents won't work properly but sometimes they do. What should I replace first before I assume that the servo is bad? Are there tests that I can do to isolate the problem? Could there be a larger problem with the brakes? (The car does stop but I practically have to stand up on the pedal sometimes) Thanks for any advice.

Jim