If you can disassemble it and get to the O-ring, you can probably determine the size by comparing with O-rings in one of those O-ring kits. If they are not completely deformed, measure with a caliper and ck ACS for comparable size.
https://www.aircraftspruce.com/catal...?clickkey=3766
the original piper valve used this series, that's what I replaced them with back a few years.