Water tank leaks only when greater than 3/4 full

[safariuser]

When I fill my tank, water starts running out of the metal support pan before it reaches full on the tank guage. I suspected a loose connection on the fill line as there is no leak when the level is below 3/4. I attempted to fill while observing the connections (on the street side of the tank) and no leaks were present on any of the connections. As I continued to fill, eventually water appeared, flowing from the street side along the bottom of the support pan.

When full to the point of leaking, and the water to the fill line shut off, eventually the leaking ceases, at I estimate 7/8 full. If you hook up and tow, I'll notice water leaking when I stop, untill approximately the guage reads 3/4 full.

I'm assuming I have a leak high on the tank, on the opposite side of the trailer from the fill. As long as I shut off the fill when the guage reaches 3/4, no leaks. As of right now, it's not worth it to me to go through the hassle of removing the tank, but I wouldn't mind having the full water capacity. Has anyone else ever run into this situation, and if so what was the source of the leak? The trailer is an 85 Soveriegn 29 footer.

[CBWELL]

Are you also assuming that the gauge is reading correctly?? Most gauges that I have had in any of my rigs are not correct. I know when my tank is full, when it comes out the overflow, or if filling quickly through the tank fill, it blows back. I don't think you have a problem.

[Belegedhel]

Well, that tank is 30+ years old...not hard to imagine that the tank has cracked, or that some previous owner drilled into it while installing flooring or fastening down a piece of furniture, etc..

The original water tank in my '73 held water fine when filled completely up, but the black tank had all kinds of cracks in it and had to be replaced.

good luck!

[safariuser]

Thanks Belegedhel and CBWELL for responding. CBWELL, the tank never reaches the designed overflow before the water starts running from the opposite side of the tank. I think Belegedhel is on the mark. The previous owner made some floor repairs and I've backed out all the installed screws I can access and I don't think more than just the tip end of any of them could protrude through the decking. Either that is enough or like you say, a 30 year old tank could develop a crack. If thats the case I'm glad its apparently on top of the tank!

[HiHoAgRV]

Check the filler tubing for cracks. My 34' did the same thing and the problem was several splits in the tube. Much easier to fix than pulling the tank!