Your pump is sucking air

Probably an air leak on the suction side.

If your pump primes fine but loses prime after a few minutes, air is getting into the system somewhere before the pump. It runs until the air pocket disrupts the impeller’s ability to pull water.

Most common culprits: a loose pump lid o-ring, a crack in the pump housing, or a small leak in the plumbing between the skimmer and the pump. The leak is small enough that water doesn’t pour out, but air gets sucked in.

Check the lid o-ring first. Lube it with pool silicone or replace it if it’s flattened or cracked. Next, look for wet spots or tiny bubbles around fittings when the pump is running. If you spray a garden hose on a suspected joint and the pump suddenly primes, you found it.

Old multiport valves on the filter can also let air in if the internal gasket is shot. That’s less common, but worth a glance.

Don’t ignore it. Running a pump that keeps losing prime will burn out the motor and make the filter work like garbage. Fix the suction side and you’re done.