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no, sorry that sounded confusing. The tractor protection valve itself has one of those crimped on vent caps. It's the same style cap like is crimped on the front gladhand covers. On my truck I was losing air out of that vent cap when applying the brakes. It also helps isolate the air leaks...

If you want to check the tractor protection valve, before you take the whole thing off, I would check the one rubber diaphragm that is located closest to the crimped on vent cap. There are just 4 screws holding that plate on and you can do that without taking the valve off the truck. On my...

Yeah those are the ones I was referring to in post #2. They are Haldex KN25060. The rebuild kit is Haldex RN11V. Not sure if that could be the issue, but I would recommend checking them either way as they all fail at some point. It's just a rubber diaphragm inside.

Have you checked the other 3 two-way check valves mounted behind the driver's side step? I'm just thinking of valves to check, and those aren't too bad to get to.