If the seals are really bad, or the piston is cracked, remember, the feed line to the brakes is higher than the axle, and siphoning can occur.
As an aside, does a Cat loafer have recircultor pumps, screens, and coolers like my later Deeres? If the recirculate pumps are hydraulic driven, could that be the source?
Is there a rear differential lock, and, if so, is it hydraulically activated? Could that be a leak source?
It would seem as if the easiest course of action is to disconnect the brake feed to the rear axle and pressurize the brakes with a porta power. Someone else will chime in if the pressure for testing should be 750 psi or not.