Originally Posted by dav7158
The cap on the end of the red drain tube appears in good condition. When I loosened it, a little water drained out .. I guess trapped in the line.
Both the low point drains (hot and cold) are simply extensions of the plumbing system in the R/V.
Anytime there is pressure in the system, either from a city water connection or the pump, there is pressurized water in those low point drains, just as there is in every other pipe in the R/V.
That is why you can drain the plumbing system from them.
Some have caps on the very ends that stick out below the R/V, some have a valve further up inside the R/V and the tube is open underneath the R/V so it doesn't freeze in cold climates.
Maybe this will help... a generic plumbing schematic.