priming and jet pumps

close friend of mine gave me a jet pump that would be useful to run some extra sprinklers, I have a well so do not use city water...

my problem is the pump does not seem to want to accept that I have 'primed' it correctly or there is something that I am not catching. I have made it pump several times after 'priming' it but other times no success.  I prime it and it seems to be OK cause the pressure is good, then I 'crack open' a spigot and the pressure disappears... the times that I have achieved success is when I open the spigot super slow and watch the pressure to try and not to let the pressure go down.  The options I can think of are the impeller is not good (this pump is 40+ years old) or air is getting in somewhere to break the suction on the pump....   ?any other ideas?    I am not a plumber by any standard...I do not know what kind of impeller these jet pumps use.    Peace, Floyd

It is a Flint & Walling pump. 1/2 horse motor

If you are using a "foot valve" only in the well DiyDave is very correct as many are not a perfect seal always. I've also seen them "foul" with sediment, sludge, or algae in the well not allowing their closure and seal. Also, a minute pinhole, or even a loose clamp will allow this suction line to drain back into the well over a few minutes, to days unless used repeatedly.

An easy option is to install a check valve into the line that is positive sealing just after the foot valve. Also, if using flexible plastic pipe on the suction line double clamp your connections using two stainless worm clamps at each connection. Myself, I always warm the connections with a propane torch before pushing and clamping the connections together. That way the clamps force the joint to conform to the connection. 

What model of Flint and Walling pump?  What well depth (for lift), and are you running a two-pipe riser, or a single? 

Frequently this kind of problem is either a configuration issue, or erosion of the nozzle coming into the pump's volute.  The volute nozzle on many jet pumps is frequently made of bakelite or similar hard plastic, and as the pump ages, incoming sand wears at it.  When the pump sits for a while, rust flakes on the inside of the housing gather, and when started up without cleaning first, those flakes get wadded up and smashed against the nozzle, making the jet pump pretty darned ineffective.  The third issue, is that one or more of the impellers is either no longer tight to the shaft, or eroded/broken away such that it's not spinning water.

The reason WHY a Jet Pump works is simple-  Centrifugal force.  There's water inside the volute, and the pump impeller stack is spinning it around in a circle fast enough to make it want to be against the outside of the housing.  When centrifugal devices fail to work, it's usually a very simple problem.

If you haven't opened it up for a look, that'd be the best step.  Flint and Walling's webpage looks pretty good, they probably have downloadable service documents on that pump.  If you pull it apart and find a damaged nozzle or spun-out impeller stack, they probably still have them available...