Proportioning Valve Replacement

Looks like you arent familiar with the Power Master PM system so heres the jist and a few pointers.

Kinda a lengthy description to help you out and also another tidbit on this topic for future searches.

As far as psi at prop valve.....

No....If there were psi in the system on the prop valve the brakes would be applied. No real psi is present at the valve (other than atmo) when brake pedal isnt pressed.

The stored psi you are thinking of is on the power assist side stored in the accumalator .

Heres the system operation in a nutshell.

basic key components...

Pump motor (motor below reservoir)
Accumalator (bowl/bulb near firewall that screwed in horizontal)
PSI switch (proper recal switch is Gray in color behind reservoir that screwed in vertical non recall is black).

Accum stores a charge that assists brake (power brakes) pedal apply and as the psi decreases from pedal apply and it lowers the stored psi in the accum to the "cut in/turn on" psi in the psi switch, that turns on pump motor .

Pump motor starts repressurizing the accumalator and once the "cutout/turn off" psi setting on the switch is reached the pump motor shuts off and the process continues.

Typical proper operation of the PM system....From a discharged state...

Key off...then press pedal until pedal gets hard.

Turn key on, pump kicks on and the engine side reservoir will drop to approx 1/4" in approx ten seconds.
During that ten second interval whatis happening is the pump is pressurizing the accumalator.

The brake reservoir is split into two halves...The half toward engine is the reservoir for the above mentioned actions.

The other half reservoir toward fender is out to pro valve and brakes.

Fluid level wise the proper way to check the engine side reservoir is..

Key pedal 10-15 or so times until it gets hard.
That has depressurized the accumalator and the fluid that was pumped into the accumalator by the pump motor is now back in the reservoir (pump didnt turn on because key was off).
At this point you will check the fluid in engine side reservoir.
Good policy to change this fluid out yearly.
I use a turkey baster and then refill with fresh fluid. You will probably notice that fluid is black.
When bleeding brakes you have the key on and will hear the pump motor coming on and off. The fluid in the engine side reservoir will not need to be refilled during bleeding.

As far as the fender side reservoir fluid level its just like a regular master cylinder and will need maintaining during bleeding.