Peter,
I don't think you could have damaged anything by pressing the pedal too far. It sounds like the problem is in the pivot mechanism between the pedal and the booster(that aluminum housing under the hood). It may be binding due to corrosion. If the booster were damaged it would leak causing the car to run oddly when the pedal was depressed. When the brakes stick, have you tried lifting the pedal up with your foot? If the pedal just flops back down when lifted from inside the car...that too would make me look closely at the pivot assembly.
Regards,
Randy D.