I"m going to "check" on the check valve. I did not remove them.
1. Tell us what year Z you have as there are differences in the hydraulic systems.
2. There are residual pressure valves in the outlets of the master cylinder. They look like a piece of rubber with a spring. Remove them.
3. You should not have two proportioning valves (PV). Having two PVs can cause problems other than what you have mentioned here. If you have for example, a 72 240Z, and you have installed a PV in the rear then the stock PV should be removed. On later model Z cars you need to understand what the PV does before removing or modifying it.
My understanding it that the stock 'proportion valve' (engine compartment) in a '72' really isnt one (only the restrictor valve in the rear)
4. The other question is why do you need a PV in the rear? 240SX rear calipers are typically undersized relative to most so called "front brake upgrades". So unless you are experimenting with high friction rear pads to fix front - rear balance issues a rear PV is not required with 240SX rear calipers.
Installed it prior to going with the big brake kit up front. I only had the rear disc brakes at the time.
5. You can relieve pressure to the front or rear brake circuits by opening the respective bleeder on the MC if you need to limp home with dragging brakes.
its up on jack stands.
6. In my experience, the front brakes will lock up after a few stops if the booster push rod is too long. It doesn't take much as the return port in the MC is small. The push rod can be adjusted without removing the MC from the car. Just unbolt it and push it to the side.
I adjusted the rod all the way back just to make sure. There is play in the rod.
7. So remove the residual pressure valves, recheck the brake booster push rod length, and road test the car.
I'll try the check Valves. I wouldnt think it would be possible for the brake distribution block in the engine compartment could hold any residual pressure, but it's the only thing I havent replaced.