I frequently have trouble getting my parking brake to engage. The big yellow handle on the dash does not seal properly when pulled. Have to fiddle with it to get it to stop leaking air. I've got the dash pulled apart at the moment (had the pull the HWH panel out for service) so I thought this would be a good time to mention this issue on the forum.
O-ring on the pull shaft (and probably other internal seals) is worn. Rebuild the valve, if a kit is available. Or replace it.
See if you can find a part or model number on the valve. Do a online Google search. See what options are available.
Parking break switch leaks all my air when pushed in (https://www.foreforums.com/index.php?topic=37582)
http://www.napaonline.com/en/p/MBIKN20021?cid=paidsearch_shopping_dcoe_google&campaign=GSC-Towing&campaign_id=8911645606&adgroup_id=106416389246&adtype=pla&gclid=EAIaIQobChMIkujticTM8QIVpRx9Ch37NAQuEAQYASABEgLMmvD_BwE&gclsrc=aw.ds&
Dan and Annie, I found this video interesting when I researched your complaint. This guy is an auto tech and admits he is not familiar with the air brake system.
The best way to see if your dash valve is the culprit is to use soapy water to check for a leak there. Most of the time you are hearing air passing through the valve to possibly another leak. A small leak else were in the rv could be your culprit. Check all your break chambers as they can be the problem when you hear that hissing at the valve. It could however be almost any thing on the primary side of the air system. Build your air up and listen. It might be a small leak so listen carefully. Yes it could be the valve it self, but many say, usually. If fiddling with the valve, as you say, stops the leak, verified with soapy water, yes it sounds like your valve is the culprit.
https://youtu.be/amAC0M2aUwA
Replaced it today with the link from Tim above. Thanks.
Note: Spent and hour with various tools trying to pound out the rolled steel pin holding the yellow handle on. Then I spied the saw in my tool kit and 30 seconds later had it off. Don't waste time trying to do it neatly.