Air Brake Leak

[ejohna2000]

1986 PT 38 8V 92 The air brake front & rear tanks are at 120 PSI when I
shut the motor off. After about 30 min. both tanks lose the same amount
of air and get down to 60 PSI and will stay there for hours. If I leave
the parking brake in the off position the loss of air is much slower.
Have replaced the check valves in both brake tanks so they can not
bleed back into the supply tank. Can you give me any idea what would
change with the parking brake off or on. I have an ultrasonic leak
detector and have gone over the complete system many times and can find
no leaks. Elmer in Sebring,FL