I wondered about the same potential problem, the push rod not seating into it's receiver every time. Upon removal of the master i could see no evidence of such slippage. Checked under the dash to see if anything funny might be happening at the connection to the pedal. All appears in order.
I wondered next about an internal seal leak within the master, so that got renewed, but alas, no joy. I did notice though, that I could not reproduce the symptom unless the booster was charged, so that's next. In fact it arrived yesterday.
I'll know by the weekend.