When I check my transmission with the motor running the dip stick indicates there is no transmission fluid but when I check the dip stick with the engine off the dip stick shows it to be full. I don get it!
In an automatic GM transmission of that era, you need to check the transmission fluid with the engine/tranny completely warmed up, on level ground, in neutral (parking brake applied), and the engine running. If it shows low, you need to add fluid. You DO NOT want to overfill the transmission, though. Add a little bit (1/2 a quart at most), then cycle the transmission through the gears slowly, ending back up in neutral. Pull the dipstick, wipe it off, restick, pull again and then check the level. If still low, add a little more and follow the same process. Overfilling an automatic transmission can be just as bad as it being under. It may cause seals to blow due to higher fluid pressures.
You need to check on/around your transmission for leaks and see if it is moist somewhere. If so, you don't want to keep running the transmission low on fluid. These transmissions are basically closed, meaning it doesn't use any fluid in the process of doing its business. If you find it low again after topping it off, you have an issue you need to have checked.