The inversion turns your park brake springs into emergency brakes if you should loose primary(rear) air. it is a spool valve with a sliding core. in one position park air passes thru to the anti-compound valve to release the park springs. in the other position it closes off the park air from the dash and vents the air in the anti-compound line out its vent(duckbill) to apply the park spring brakes. the sliding core has primary air on one side pushing to spool core one way(allows park air to reach the anti-compound). It samples secondary pedal air on the other side of the spool. if secondary pedal air is greater than primary tank air(lost primary/rear brakes), that secondary air pushes the inversion spool to vent park air and apply the spring brakes with the secondary when you push the pedal(~50% rear braking force).
Our trucks are wired with 2 idiot lights, one for PARK and one for EMER dash lights. But they only put in the park switch and wired both dash lights to that one switch. they should have put another switch on the park air output side of the inversion valve and connected it to the EMER dash idiot light to tell you when the system/inversion valve was employing emergency braking ...
If you have good primary air and the only time it is passing air out the vent is when you apply park air, i suspect the spool seal is simply leaking across to the vent port. input park air should never be able to reach the vent port anyway, so pull it apart and overhaul it(should be able to get an overhaul kit from Haldex) or replace the valve completely...
Good Luck.
