Trans problem, need advice

The Park and Neutral circuits are the same except in Park, the manual valve dumps fluid. This reduces the line pressure from something like 55psi, to half or less. The lube circuit, is still active, as is the TC charge circuit. But at idle, not enough oil flows into the TC because the manual valve is dumping most of it into the pan.
However, the 69 FSM shows a TC check valve, which is supposed to prevent TC drainback. I mean I guess cuz why else would it be there,lol.