Trailer one way Valve

[Waterloo]

Where exactly are you hearing the swishing noise? In the dash at the brake valves?

Lines at the dash valve gunked up? When was the last time you changed the air cartridge in the air dryer? Bled the tanks?

I know every once in awhile I get air leaks in the dash valve, it will bleed air and then eventually stop. I have to build up the pressure, set the brakes, release the brakes, and it quits. I have pulled out the valve, cleaned the lines, etc. It still does it once every say four five months, and goes away... No clue on my end. I change the air cartridge and drain the tanks, etc on a regular basis...