That's the test, pressure on the pedal and if it sinks it's faulty. Do it with the engine off otherwise it's inconclusive between the MC and the booster, with no boost the pedal should be very firm and have no movement. Toyota calls the pedal movement after the pads and shoes engage "reserve travel" I think and the spec is like 3mm. Sounds like to me it's bad.
'91 Pickup - Imelda
'08 Tacoma TRD - Donna