Passat (B5) :: Inefficient Braking Action Especially In Reverse

Here's an interesting problem our B5 (2001 1.8T) has experienced recently -- When backing up and then applying the brakes there's very little effective braking occurring, sometimes to the point of the pedal traveling almost to the floor. I've recently had the brake pads and rotors checked -- all rotors are OK and the front pads are "almost new" condition. The rear pads were showing significant wear and I had those replaced, but we're still seeing the same problem.

I can also tell that braking action driving forward feels less than optimal, although not nearly as troubling as when attempting to stop in reverse. I'm suspicious of a problem with the vacuum assist unit, either a bad hose or possibly a defect in the boost unit itself. Can the VW shop definitively test the appropriate vacuum line(s) to the booster and the booster unit itself for a problem?