Firm pedal when car is in reverse? It sounds like you are holding the pedal down before putting the car in reverse, then your booster is empty (or full of air...) and it no longer functions. I bet if you hold the pedal down in forward gears, it will be hard as well.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!