On May 10, 7:44 pm, wrote:
I pull water from a lake and pump it up to my cottage. For some
reason, my pump has started short cycling. I changed the pressure
switch, and the pressure tank is fully charged (about 38 lbs). It is
primed. If I open the prime port and turn the pump in it shoots water
high into the air - getting water is not the problem.

When the pump comes on, it quickly builds pressure to 40 lbs and then
shuts off, pressure immediatley drops to 20, pump comes on, then off
very rapidly. Would a leak in the line between the jet pump and the
house cause this? I am thinking it might have frozen and split....

Others have covered the problem and solution. I am just wrapping it
all up in one:

I am assuming the pressure tank is in the house:

1. Your pre-charge in the tank is too high. Drain tank and re-charge
to 18psi. If that doesn't cure it, then you have a leak.

2. The leak will be either in the inlet pipe to the tank. It can be
a broken pipe, a bad footvalve or a check valve. You can fix the
problem by installing a new checkvalve just prior to the tank. The
system will work just fine as long as the pump doesn't loose prime but
if it is a broken pipe you will be pumping a lot of water out of it
every time the tank fills. May not matter

Now if the tank is co-located with the pump, then you have to find the
leak and fix it. Start witht he foot valve. If it is in the pipe
going to the cottage, you'll have to dig it up and fix.

Harry K