My new aftermarket disk/disk prop valve failed in the same way both of yours did when I tried it in 2017. The brass style aftermarket ones are direct swap overs to the factory G body cast iron Kelsey-hayes or Bendix supplied ones ( I forget what style the G body has but almost all the OEM valves are originally supplied by one of those two manufacturers)
Either the Bendix or Kelsey-hayes can be 'gutted' and converted to run disk/disk by removing some spring and ball, but the G body factory valve is the type that can't be converted. The aftermarket style is a clone of the type that can be converted. From what I gather it's just missing the drum pressure reducing parts that the disk/drum style has.
Potentially inline tube or other 'premium' distributors have some test that they pay for that makes them better than the elcheepo fleabay style but again, I haven't seen any evidence they are supplied by different manufacturers. The design is the same and volumes are low enough I highly doubt there is more that one oversees manufacturer.
If you are firm and want a replacement that will 100% swap an NOS ws6 f body part is probably your only shot.
"1978-81 FIREBIRD TRANS AM 4 WHEEL DISC BRAKE VALVE NEW GM NOS OLD STOCK 1257189 | eBay"
https://www.ebay.com/itm/3627415265...d=link&campid=5335822911&toolid=20001&mkevt=1