PT Lacrosse 301RLS Black Tanks

[Walke]

My 2012 Prime Time Lacrosse 301RLS has a Black Tank Flush system. After the winter it does not flush. Instead, water floods the bathroom floor from a black plastic device on some tubing located under the sink with the pump, winterization tubing, etc.
I don't know what this device does, when I apply black tank flush water it comes out of this device instead of flushing the tank ???.
I will fill the Black tank with water and an enzyme treatment to soak in case the flush bar / aspirators are plugged, but where is this tubing under the sink connected in the flush circuit??

[ependydad]

It's a 1-way check valve. Supposedly they're easy to replace or even remove entirely.

[Learjet]

yep froze and cracked anti siphon valve