CSY 37 water tank vent problems

[Red Sky]

I have a CSY37 with Fiberglass water tanks on both sides under the bunks in the Salon. When I fill them, when they get to 90-95% full they swell up like a balloon with no water or air coming out the overflow vent. There is a 5/8" vent overflow about two inched from the top of the tank on the forward side next to the Fill that goes to outboard the fill pipe on the Deck. They start to Balloon after the water level reaches above the vent with no air coming out the vent! It seems that there is a air pocket that is not venting causing a air lock because it it trapped due to the water in the vent hose. The tanks are thick fibreglass and strong. It is causing all kinds of stress problems / leaks that we have done fibreglass repairs on. I think I need to move the Vent higher so it can breath properly but can't put it out of the top because there is no room. It seems that the Air pocket at the top and water in the vent hose is causing the problem of the tank not breathing thru the vent when it get 90-95% full! Should I put a vented loop in ? but I don’t see how that would solve the problem! have anyone had any experience with this? Any body with any Ideas would be most appreciated???

[Matthew Balch]

It sounds like your vent pipes are clogged or crimped.  Bees and other insects have been known to build nests in vent pipes.  I can only suggest that you disconnect the pipe from the tank and try to blow it out if you haven't done so already.

[TattooLuke]

I think you are figuring right.once the water level reaches the vent then your trying to push water out the vent. Maybe a small tube on top with a T to the larger vent.my tanks are aluminum did yours come in fiberglass fron the factory?

[Rhapsody]

Had the same problem on my CSY33, the solution was to literally rock the boat, then the air would escape and I could fill the tank all the way up.