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Jon is spot on… The leveling valve is out of the loop because the Norgren valves “lock” all air therefore your front end drop is the result of leaks elsewhere:
The two Norgren valves can leak internally and not be detectable, its fittings, the air bags themselves and their fittings, the ping tank itself, its fittings and drain valve. Also, the top Prevost installed Norgren on back wall of steer bay – it can leak internally or its top fitting.
Those three norgren’s, the two airbags, the two ping air tanks and ALL attached fits & drains are the only components that can leak causing a front end to go down with the key off.
****Question for you… Were the fittings you found to have small leaks… were they push-in or compression and how did you resolve their leaks?