Thanks for the suggestions! I just got access to the car and listened to the booster on the firewall above the brake pedal. When I start the car and press on the pedal, I hear a whoosh or hiss sound each time as it moves. I assume I shouldn't hear any air noises if the seal is still working properly...

Thanks, jonesg. I'll start there.

Volvo wizards - My 2.5T vacuum boost pump runs (buzzes loudly) for 1-2 seconds every time I step on the brake pedal. This happens whether the car was just started or has been driven for many miles. Twice the past year, it started buzzing and wouldn't stop until I cut off the engine. The brakes feel ...