Classic/Antique Car Repair: 1969 Camaro Transmission, Park gear doesnt hold car

Question
3-speed automatic transmission with floor/console shifter.
Car will roll backward when shifter is in "Park" otherwise seems to be working fine.  Ideas for problem, it should not roll when in "Park".

Answer
There are only two possibilities here;  either the parking lock mechanism inside the transmission has failed, or the shift linkage isn't traveling far enough to engage the parking lock.

The way to sort this out is to disconnect the shift lever mechanism under the console and manually move the external shift control on the transmission all the way into the "park" position, then see if it holds the car stationary. If it does, your problem is in the shift linkage, or possible a bad motor mount that has allowed the engine/transmission assembly to move away from the right position.

If it still doesn't hold the car when you manually put it in park using the transmission control shaft, then your problem is internal to the transmission.

Dick