Help please, Lake water line leak

I was wondering if anyone had any ideas for how to find a leak in a water line drawing from the lake. It is an 1 1/4" potable water line that is insulated and covered with a corrugated plastic pipe over it. There is an internal heat tracing line that is about 150'. The water line is about 250' long. The insulation runs until it gets to the water level and a little below. Pump is a 10 gpm piston pump working fine.

We opened this year about six weeks ago and got the water on 4 weeks ago. The first week was fine. Since then I am sucking air in and lose the prime on the pump. I build about 6psi of pressure before losing the prime. I have dug a bunch of it up and have inspected a section that runs hung up under the dock. While dug up I have pumped several pressure tanks of water back into the suction line. Approx 10 gallons each time. I have hooked a shop vac up to the suction line and looked and listened. No bubbles in the water although lots of ice still out there. No noise on land.....

I looked on line and it looks like the water in the 250' line is about the amount it would take to fill the pressure tank. Total psi differential is about 14 psi. I figure getting 6 puts the leak a ways from the cottage but not out past where it heads under water. If it was out there I would have seen bubbles when I had the shop vac hooked up.

Any ideas on how to narrow down the search area?

Thanks in advance for any replys.

Neil

I would suggest that if you had water for 4 weeks, the main water line is not the culprit, maybe the foot valve at the end of the hose. If there was a split anywhere above the water line, there's not a hope in hell you would suck up any water.

Methinks he nailed it . Neil . i'm not a licensed plumber but he nailed it where I would start .IF that fails and pump is good . Don't waste your time looking for underground leak ,Run new line . Don't ask how or why we ran over a 1000 feet 50 years and double . You have two check points other than pump and got good advice

TD

Thanks for the feedback gents. Will be a couple weeks till I can get to the foot valve.
Interestingly my neighbour uses a system with a check valve at the pump and just a strainer at the pickup end. Never has to worry about changing a check valve which I do in the fall every year. The system has to be tight though.

Anyways thanks again.

Neil

If you have a Schroeder valve on your pump to add air to your tank make sure it’s not sucking air..... I had that happen once. Try a new foot valve,if you have the same problem,go with previous advice and run a new line......it’s not worth trying to find a leak in 250 feet of line. Hope it helps and good luck.

Ah, if the schraeder valve was leaking and sucking air in, the pump would have to be running backwards. Piston pumps don't work backwards. I've never seen an air valve on the pump, only on the tank.

Thanks for the thoughts folks.
No schraeder valve. Sounds like the foot valve is the likely candidate.

Neil