Sonnax manual valve for A500 transmission.

The stock spool valve on a 904/727 will allow convertor drainback in P, not sure of the 500 series but most likely. The NO-YO-YO kit from Gil Younger (TransGo?) is just a new spool valve that has 1 land modified so in P the convertor wont drain back. If its just a valve body, it can be done on your back in the car. Pull the pan the night before and crack the valve body screws so it can drain in the pan, not on you.