My headhunter pump was cycling and I had to purge some air out of it and noticed the same thing, no place for the water to go. I was thinking about adding a drain in the bay so the water has a way to get out. Looks like the pass through for the water hose is nothing more than a sink drain fitting, I was thinking about adding another one of those and adding a screen to it to allow any water spills to drain from the compartment.
I had the same problem so I drilled a 5/8" hole through the wood and foam then took a piece of plastic tubing and cut it to the length that it stuck out the bottom 1/2" and used silicon rubber them in place. Looks like crap but works great, I wanted the top of the pipe even with the floor.
On our airstream interstate class B we had a electric dump valve and it worked great, I did away with the solenoid valves and the air dump valve as its just one more place to leak air.