I have a firefighter pump that I use to draw water out of a bore about 16' under ground. After not being used for some time, when I went to use it, the engine went ok but no water was being pumped out of the bore...as it has always done before, over some years.
I took this pump out and replaced it with a less powerful one, and the bore system worked very well....so my assumption was that something is wrong with the main pump.

I spoke to the mechanic from where the pump was purchased from and he suggested that I replace the large "O" ring rubber seal and also the seal on the inlet, which also includes the one-way 'flap'. I replaced these and pump still didn't pump water, after priming of course.

I then took it to the mechanic and he said I had placed the 'diffuser' that goes over the impeller in the wrong position, so he fixed this and tested pump and all was ok.
Unfortunately after picking up the pump it still didn't work, so took it back again.
The mechanic tested the pump by placing an inlet hose into a 44 gall drum, started pump at similar level to top of tank, then lifted the pump up about 4' above water level and pump seemed to draw water ok...as there was limited water to test it with, it only could be run for a very short time as the inlet/outlets are 1.25" diameter.

I re-installed pump again, and what do you know, it still isn't pumping water...tried smaller pump and all works well.
Neither the mechanic or I can work out what the problem may be, hence this forum....help!!
I know that air leaks in the incoming pipe can cause the pump not to suck water, but given that the smaller pump works on this same system, that seems to negate this as an issue.

I would appreciate it if anyone has any other suggestions on what the problem may be...my apologies for the length of the message but I have tried to outline as much detail as possible in the hope that someone may have an answer.
Many thanks