I put a new pressure tank in came precharged at 39 psi. Also put a pressure switch in 30/50. Dropped tank to 28 psi. Pump would not pump up to 42 psi. So I adjusted the switch to 20/40. Then pump would not pump over 34 psi. So inturn I let more air out of tank then switch would drop out. When air added in tank to 28 psi the switch would not drop out and cut in at 17 psi and out at 33 psi. I know this is wrong way to set everything up but it's the only way I could get the pump to run at a normal amount to get a break. The Well is 75' and a one pipe system. It's a customer of mine and they really don't have the money to throw around. The pressure is enough to run the sink and shower with just a little notice of the pump kicking on when running. I was thinking maybe there is some dirt brought up and blocking some of the inlet to the pump or their pump could be on its way out. Any ideas anyone?

1. What problem were you trying to solve when you put in the new tank and switch? Was the old tank waterlogged?
2. Assuming that the pump is above ground, then this is one of the most common complaints we see about wells, a shallow well pump that cannot get up to cut off pressure. I would start over. If the switch is now set at 20/40, then make sure the tank is set at 18 with power off and system pressure drained to zero or at least near zero. Just be aware that as the pump delivers water, the water level in the well is probably dropping. If it drops too much, then the pump struggles to build pressure. It cannot pump efficiently from much more than 25 feet of depth. Even if the well is 75', you can't get water from much more than 25 feet, so you might end up having to shoot for more like a 20/35 setting. You can adjust the cutoff pressure only by turning the smaller nut.

http://inspectapedia.com/water/Pressure_Switch_Nut_Adjust.htm