One small valve keeps your entire system pressurized between pump cycles. When it fails, the symptoms look like a dozen other problems — here's how to tell it's the valve.
The check valve is a one-way valve that keeps water from flowing backward down the well after the pump shuts off. Without it, water in the pipe above the pump would drain back into the well every time the pump stops, and the pump would have to re-prime and push that column of water back up on every single cycle. Most systems have at least one check valve at the pump itself, and deeper wells often have additional in-line check valves every 100-200 feet of pipe.
Quick test: Shut off the pump breaker, open a faucet, and watch the pressure gauge at the tank. If pressure drops to zero almost instantly rather than bleeding down gradually as the tank empties, a check valve is likely stuck open or has failed.
On a submersible pump, the first check valve is usually built into or threaded directly onto the pump discharge, meaning a failed valve often requires pulling the pump — a job most homeowners hire out. Additional in-line check valves along the drop pipe are more accessible if you have record of their depth from the well installer.
Rapid pressure loss and short cycling can also mean your pressure tank has lost its air charge. Check the tank's air pre-charge first (with power off and tank drained) since that's a 5-minute fix, before assuming you need to pull the pump for a check valve.