pump won't prime

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Feb 4, 2015
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Hi All- I'm in a real pickle. The water system shown is in a mobile home placed in the 60's. The "well" was dug directly under the floor and is only a few feet from a brook. The inlet pipe in the photo passes through the floor and goes right into the ground. I'm in VT and not sure I can get under the unit due to snow. The water system has been working fine for years. About 5 years ago, the foot valve started to fail and the pump began cycling occasionally to recover pressure lost from drain down. I installed the brass check valve shown in the pic I've attached, and the system has been working fine now for years. The building is occasional use. It is winter here and I've been working on it and running heat and using the water (filling many buckets, flushing the toilet, etc.) My usual routine is to drain the plumbing in the fall and empty the pump housing through that little ball valve you see in the pic. to avoid frost damage. Priming the pump is a bit tedious in the spring but works every time by gradually filling the pump housing, replacing the bleeder plug, running the pump- alternating this way until the air is worked out of the housing and I get that satisfying "whoosh" as the pressure tank fills up. It's been cold lately, so I've been turning the pump off and depressurizing (not emptying) the system when I leave at night, so if there's a freeze up I won't get flooded. I thought emptying just the pressure tank this way was a good strategy so there's no residual water pressure in the dwelling. I've done this numerous times this winter and each time when I turned the pump back on the system has pressurized without having to re-prime the pump. Except today. I keep finding a bit of air at the top of the pump housing after repeated tries. I can get the pressure as high as 28lbs. I then shut off the supply ball valve left of the gauge and turn off the pump. Each time I do this I remove the filler plug at the top of the housing. Each time there's a slight vacuum hiss and room for more water- about 1/4 cup, like when I prime it in the spring. I top it off, replace the plug and try the pump again. By leaving the supply ball valve closed I can keep pressure at 28psi, but it won't advance. After this 28 lbs is built up and I shut off the ball supply valve and re-fill the pump housing, the pump even starts to short cycle until I open the supply ball valve again, then it runs continuously and the tank doesn't build pressure past 28psi. Can anyone tell me what the problem might be? Thanks, and sorry for such a long explanation!