I'm usually pretty good at fixing things, but know nothing about plumbing and this well setup has me puzzled. A little background. The well is used only for an irrigation system and an additional faucet on both the front and back of the house. I've been in the house 7 years and everything was here when I moved in. The pump and well has worked fine up until a couple weeks ago. The pump is a Sta Rite SNE-L jet pump with a 30/50 switch.

I came back from a vacation and the pump was running with no pressure. Not sure how many hours or days it had been running, but was surprised it hadn't overheated or tripped a breaker. Unplugged it and tried to figure out the problem. No noticeable leaks. Checked the tank with a tire gauge and it had 0 psi. Got out the compressor and filled it to 28 psi.

This is where I'm not sure if I'm following the correct procedure. In the attached picture there is the blue "regular" faucet round valve, a black valve on the pipe, and an octagon shaped plug on the pipe going from the pump back into the wall.

I thought maybe there was a priming issue. I tried hooking up a hose to the blue valve, opening it up and running water into the system. I've also tried adding water into the octagon shaped plug. I've even tried various combinations of open and closed blue valve, black valve, etc and can't get the system to work correctly. The pump will get to about 20 psi, but no higher.

Can someone tell me the purpose of the black valve? Shouldn't is always be open? When woukd I use the blue valve? What's the plug used for? Priming? What would be the correct procedure for getting everything back up and running? Could it be a bad bladder in the tank?

As you can see, I'm really stumped and feel like I've tried everything. Thank you very much for any info and help.