auto transmission slips in Park

I've been saving up my questions :)

Occasionally when parking on a grade, the transmission of our 90 525iA will not lock completely; the car will creep backwards. If the grade is severe enough, the car will start to roll (found that out while sitting in the passenger seat- thank god the engine was running!)

From what little I know of automatics, there is a pin that locks the transmission to prevent creep/rolling when in P(ark). Is this repairable without tearing down the hole tranny (prolly not)?

Of course this wouldn't be such a big deal if I had adjusted the parking brake correctly during the rotor and pad replacement....

Cheers,

Roland