The guys at Muscle Car research sell rebuild kits for the pressure differential valves 67-75 GM.
There is one for the plastic terminal and one for the steel terminal.
Look here:
https://www.musclecarresearch.com/gm-pd-valve-kit-1
The extra one I have is for the steel terminal. Also, I do not know if this will fit a replacement valve that may be made of brass. My original is made of cast iron.
Free to first guy or gal to send me a pm with your address.
Lynn