Since it was still a balmy 38º I ventured down to the garage to try what TJP had suggested.
I pumped the brakes for a bit, then held my foot on the pedal with steady pressure and turned the electric vacuum pump on. The pedal did not move.
Assuming the above test was done with steady pressure on the pedal, And
that the (the vacuum pump) ran until it shutoff (15" +), And
the pedal did not drop with the same steady pressure applied, That would indicate a bad booster. It may also explain the "groan" noise you described.
The booster is supposed to multiply the pedal pressure applied by using the vacuum. The above test should cause the pedal to drop as the vacuum is applied thereby generating more hydraulic pressure with same pedal effort, hence the term "power brakes".
hope this is making sense
