Some air escaping from the exhaust port is normal as the hcv attempts to keep the coach level during varying loads. I would advise spraying the whole valve with soapy water to check the seals and fittings.
1993 Newell (316) 45' 8V92,towing an Imperial open trailer or RnR custom built enclosed trailer. FMCA#232958 '67 Airstream Overlander 27' '67GTO,'76TransAm,'52Chevy panel, 2000 Corvette "Lingenfelter"modified, '23 Grand Cherokee.