Brakes works good, but air parking brake does no work. When parking brake is engaged, it sounds like it will work, but it will not hold the cat. Is there a way to adjust an actuator or something so the parking brake will hold?
Serial number would help but I believe you adjust the clevis on the engagement rod to apply more pressure. A lot of times those machines got driven with the brake on destroying the brake linings.
1981 Cat 950A Serial # 73J9925
I would strongly suggest that you take a look inside the parking brake first and see whether there are any brake linings left before you start attempting to adjust it. Attached is the disassembly & assembly procedure. Also attached are two procedures for adjusting the parking brake. Use whichever one corresponds to physical arrangement that you have on your loader.