The water pump runs continuously even when all the water faucets are closed. I thought it might be the pressure switch which is mounted on the end of the pump and changed the switch to a new one. No joy! So I changed the pump out to a new pump (different brand) I carry as a spare. No joy! There is no water leaking from any fitting or line, the flow thru the faucets is ok (always weak flow using the pump), and I removed the water filter cartridge and checked the pump suction strainer (clean).So why can't the system build up the 50 psi pressure to shut off if there are no leaks and the supply is unobstructed? Stumped in Washington!

If there are no leaks on the output side / lines then the pump must be sucking air on the intake side. Maybe try blocking the output side near the pump and see if it will build up pressure.
It is interesting you say the water flow is always weak when running on the pump, ours actually has pretty good flow when running on the pump.

It is a rather short run from the pump to the fresh water tank which is full. I'll run the winterizing line which is on the suction side of the pump in a bucket of fresh water and see if there is any better flow and pressure. Mind you the system was operating normally when I winterized it in October.

We do not use the water pump all that much, only when at a rest area bathroom breaks or when it gets too cold for our fresh water line to be used. (which is typically only a couple of times a year)
The water pump has always preformed good, but we never winterize since we live in our Redwood fulltime year-round.

Problem solved. The plastic cap on the anti freeze pick up hose was leaking air into the system. There is no valve in this line so it depends on an air tight seal from the cap. One of the first things I checked was this cap and I screwed it on as tightly as I could. I found the problem when I removed the cap and inserted the antifreeze hose into a jug of fresh water. When the line was trapped under water the pump pressurized the system and then cycled off from the pressure switch. The solution was to insert a rubber washer into the cap and also use teflon tape on the threads. Everything is back to normal. 😛

Guess I was on the right track with sucking air on the intake side 🙂

Guess I was on the right track with sucking air on the intake side 🙂

You certainly were and thanks for that. I plan to install a valve in that line.