Am I diagnosing this well pump problem right? If so, buying new pump in am.

Water system pressure has been weakening for years, so that whenever I use even a couple gallons of water the pump cycles for a minute or so. Last Thursday I had no water and the pump had lost its prime. (Note: ancient, possibly 50-y/o Goulds 8165 1/2 hp jet pump in sub-cellar, well depth around 25 ft, piping is 1.25 in. cast iron). Lots of possible problems at this point.
1. Tried to re-prime pump, got weak water pressure, never over 32 lbs. and pump stayed on for 1/2 hour even with no water running in house. I shut it off, and it lost prime again. Tried to fill it up but water bled back down into well. Diagnosis: bad check valve.
2. Saturday, replaced check valve and connecting nipples on either side. Old nipples had crumbled, rusted threads and there were big bits of rusted iron in the old check valve. Made sure that new valve was installed in the right direction. Also installed a bit of clear pipe between pump outlet and copper pipe leading to pressure tank. Pump took and held prime, but would not build pressure and moved no water. There are air bubbles going through the clear pipe. Figured there was air behind the check valve, removed check valve and backfilled piping into ground from neighbor's hose. Reconnected everything, same problem. Possible diagnosis: old union leaking air?
3. Today installed new union and nipple leading directly to pump, and new pressure gauge to ensure I'm getting the truth. Now all piping from elbow before check valve to pump is new. Pump takes and holds prime and is moving water but doesn't exceed 20 psi and loses that after a few minutes. The clear pipe shows a few air bubbles for the first minute or so but then a solid column of water moving through, with hardly any bubbles. Water flows weakly from spigots on first floor, fortunately enough to take a quick shower. If a first floor tap is shut off and then re-opened, pump loses pressure to down below 10 lbs. and bubbles reappear in the clear pipe. Diagnosis: pump is bad, need to buy new pump.
What could I be getting wrong here - are there other possibilities? New pump will be $300, if the real solution is cheaper, I'm all ears and very grateful to the correct adviser.
Thanks.
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My vote is an eroded, worn impeller.
RJ

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I would check into getting the Gould rebuilt. I had a Gould, lasted 13 years. It was replaced with another brand that lasted 3 years and was crappy during that time.

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When my pump lost prime it was due to a leak in the suction piping.
To test the pump, I would turn all faucets off. Put the suction into a 5-gallon bucket that is being filled from the neighbor's hose. See if your pump will build pressure into the pressure tank. If it does NOT, bad pump. If it does, run a hose from the pressure tank back into the bucket and see if it will keep up with the draw from the hose. If it does, probable well or suction piping problem. If it does not keep up with hose, probable pump problem.
Stretch
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Thanks to all. Spoke with a plumber last night, he suggested it might be a clogged jet in the pump, cleaning which would be an easy fix. But I won't know anyh time soon, because the bolt covering the jet access broke off when I tried to remove it to clean the jet. I'm not equipped to remove a broken bolt, so I'm buying a new pump this morning.
I'm pretty sure the old pump lasted at least 40 years, possibly more.
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Bobn,
Check valves either work or they don't. Your slowly worsening symptoms sound like a leak that is slowly getting bigger. It doesn't sound like a bad pump since you are losing prime. If you don't see an obvious wet spot between the well head and the house then a good bet is that the leak is in the well. Replace all metal parts, unions, and pipes in the well and see if that fixes it, If you still lose prime then you need to find the leak between the well head and the house
Dave M.
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Right, Dave. Since the original post the pump continues to hold prime with no problem. But the pump can't build more than 20 lbs pressure. I think the impeller is probably shot. I'll be installing a new pump after work today.

bad
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