Goulds GT15 and lake irrigation system - intake problems

[Coastalcamera]

I'm having some issues getting a Goulds GT15 pump sucking water from a fairly clean lake. The pump is ~20 years old and the suction line was redone my a seawall company in 2017 and stopped sucking water last year. I rebuilt the pump using a Goulds rebuild kit with new impeller, guide vane, neoprene seal and the rest of the kit. When I took the old impeller out there was a small pebble stuck inside. The line is 1.5" and ~125 feet in length with a 6 foot lift from the lake and probably travels up about another 2-3 feet over the length of the horizontal run, there was also a pvc spring, not flapper, check valve that was horizontal after the original lift at the lake side. They also used what looked like a pvc shower drain.

I have replaced some parts piece by piece and now have a a foot valve several feet below the lowest water level and still several feet off the floor of the lake, a pvc union near the top of the vertical lift from the pump and a union about a foot away from the suction side of the pump. I have primed the line both from the output side of the pump with a garden hose connector installed and have also filled the suction line from the union on the intake side. Whenever I start the pump and use the hose outlet on the output side before the manifold probably a couple of gallons come out over several seconds and the water stops flowing and totally stops then I'll shut the pump off. If I crack the union near the intake I can hear what sounds like a strong vacuum once the connection is broken. PVC has been purple primered and Oatey pvc glued with megaloc tape and some rector 5 at the threaded connection going into the pump. Unions are glued and not threaded. I have tried dozens of times and I'm not sure if I'm not priming correctly, if there is air in the line or the pipe has been damaged creating the original issues or what is going on. I filled the intake line with water last night and 16 hours later it was down maybe 1" in a vertical line by the pump, just a very slight amount.

Any advice or additional trouble shooting would be greatly appreciated. I'm happy to provide any more details or info that may help. Much thanks!

 

[WorthFlorida]

Remove the foot valve and give it a try. You only need one check valve. With everything going on is it possible the check valve was installed backwards?

If still no water something maybe jammed inside the intake pipe. You have a rock solid irrigation pump and doubtful its the problem since you feel or hear a suction when opening the union.

In Florida most use a intake filter screen with a PVC check valve near or under water. https://www.homedepot.com/p/Water-Source-2-ft-Canal-Lake-Screen-WSCLS300/308718009

 

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[Valveman]

I have the hardest time making PVC unions hold a vacuum?