Tank pressure goes to zero

[Allen S]

I have a Challenger Series diaphragm well tank Model #66. I have a Square D Pumptrol pressure switch. When I open a faucet the gauge goes down to around 25 psi then clicks on and boosts to 50 psi. In about 10 seconds it drops back to 25 psi and repeats continually until the faucet is closed. The water pressure at the faucet constantly goes up and down. I drained the tank and checked the pressure. It was zero. I closed the faucet and pumped the tank to 29 psi. Before I turned the pump back on, I opened the faucet and the pressure immediately began to drop. I closed the faucet and pumped the pressure in the tank back to 29 psi. In about 15 minutes it was back to zero with the faucet closed..
Also, the pressure switch has a knocking sound when it is running. It has been doing this for over a year.
What do I need to do, and what can I expect the cost to be?

Thanks, Allen

[Rancher]

I closed the faucet and pumped the tank to 29 psi. Before I turned the pump back on, I opened the faucet and the pressure immediately began to drop.

I'd say the bladder is leaking, time for a new tank.

Also, the pressure switch has a knocking sound when it is running.

Pressure switches don't run, so I assume you mean when the pump is running... not sure what is causing the knocking, are you sure it's not the pump making that noise?

What do I need to do, and what can I expect the cost to be?

That depends on if you can do it yourself or not, for quality tanks and pumps go see Speedbump at http://www.pumpsandtanks.com

Rancher

[jadnashua]

If there is no air in the bladder tank, the shock of the pump turning on and off can cause the pipes to be shocked. You might have taken a few years off your pump by not addressing this sooner, too.

With no bladder, every time you open a faucet, the pump needs to turn on to maintain the pressure.