Dribbling water tank vent line?

We have a 1996, Mainship 370 Motor Yacht (not to be pretentious, that's what they called it). Big white tennis shoe style, not a trawler style. I have four vent caps on the hull. One fuel and one waste, about midship port side. One water vent, midship starboard side. I replaced all three of those last year so I know how they're plumbed, nothing complicated, just vent caps, single hoses to each tank. However, I have have a water tank vent cap port side aft, right next to the swim step stairs. I can't access the back side without dismantling the wall in the aft berth, so I don't know how it's plumbed. So here's the question:

Why on earth would water dribble out of that vent cap when I'm connected to shore water? The water tank is only half full. Steady dribble, maybe two gallons an hour. I turn off the shore water, it stops. Does that line drain the ice maker on the upper deck too maybe? I'm stumped, without taking out the berth wall. Thanks.

How do you know that it's a water tank vent? Does it 'gush' water when the tank becomes full while you're filling it? I'm wondering if it could be an over pressure outlet for the shore-connected water system that leaks a bit. Just a guess.

Greg.

Or maybe shore pressure water is overpowering a leaking valve or pump. How about draining the tank to halfway and then see if it gets magically refilled while shore pressure is on?

Oh I know that vent cap is the port water tank, per the Mainship manual and I've seen the plumbing for the other three, so I know for certain where the other three vent caps go. Tank is half full, even tried taking the deck plate off to make sure it wasn't a pressure/expansion thing on a hot day. I do have a (relatively new) pressure reducer on the shore water inlet, but even if I didnt, I don't know how high shore water pressure would force water out the vent cap in a half-filled on board tank. Hmm.

Are you sure your water tank is only half full? There should be a check valve or perhaps a manual valve that prevents shore water from back feeding into the water tank.

Ken

Just checked again - I filled it only halfway last week, but you're right, I unscrewed the deck plate and turned on the hose to see how much it would take - it is full, overflowed in three seconds. Thanks everybody, yep, that's the problem. Now I have to figure out where and how to get to the plumbing inboard of the water locker on the swim step. Thanks so much again. Always something!

water pump should have a built in check valve but that may be leaking on shore water. Have you checked the pressure of the shore water? Our marina has 100 PSI and causes problems if we don't use a regulator. You can get inline check valves that fit the PEX fittings so adding one inline is fairly simple.

I do have a relatively new pressure regulator on the incoming shore water, I'm pretty sure that's okay. I'll bet it's the check valve on the tank. If it's actually under the aft berth as this diagram shows, access might not be too difficult. Diagram attached -- I'll bet it's that check valve just to the left of the 80 gal FW tank. One thing I've learned from these Mainship manual diagrams, they're schematic only, that check valve could actually be physically located -- heck, in the anchor locker in the bow or in the radar dome overhead for all I know but I'll pull the mattress next time I'm on the boat and see. If it is under the mattress though, that'll be easy. Well, to the extent anything on a boat is easy.

Pulled apart the aft berth, dragged it all out to the salon, took out a million screws to remove the aft berth platform board, uncovered the 80 gal port side water tank -- sure enough, the check valve on the house supply line was shot. The internal parts were completely gone, the bits are probably lying on the bottom of the water tank. The check valve was just an empty tube. Ordered a new check valve plus a spare and will button everything up next weekend. Thanks very much everybody, sure appreciate the advice.

Maybe put an access panel above where the check valve lives so next time you won’t have to disassemble hundreds of screws. Just a thought...