Bendix brake valve question

[Pro Star Cummins]

Not sure ,but is there a small check Valve back by rear end housing
Along frame rail? I think there is a slinding part in there that gets stuck
In one position and allows air to bypass ,I had similar problem
Along time ago and that is what I found. Replaced brake treadle and
Quick release valve, but still same thing was driving me nuts till
I discovered that valve. Changed it out and that fixed the problem.
Google Bendix 2008 Pro Star Truck brake diagram .
Sorry I don't know how to load that address directly.