A pump that loses prime repeatedly is telling you there's an air intrusion point somewhere on the suction side. This is distinct from a pump that won't prime at all — losing prime means the pump primes fine initially, then over hours or days the prime erodes as air migrates into the suction plumbing. Tracking down the source requires a systematic approach.
Air leaks only occur on the suction side — the plumbing between the pool and the pump. Pressure-side plumbing (between pump and filter, then back to pool) leaks outward (water drips), not inward. If you see water dripping from a fitting on the pressure side, that's a water leak, not an air leak causing prime loss.
The suction side is under negative pressure (vacuum) while the pump runs. Any imperfection in a fitting, O-ring, or joint allows atmospheric air to be drawn in, reducing the pump's ability to maintain suction.
Work from the pump backward toward the pool, checking each potential leak point:
With the pump running and struggling to hold prime, apply a thick coating of shaving cream to each fitting, union, and connection on the suction side. Watch for the cream being visibly drawn into a fitting — that's your leak. This method works well for above-ground fittings and unions.
Watch the pump basket through the clear lid. Air bubbles rising steadily in a consistent location suggest the leak is close to (or at) that side of the pump. Bubbles scattered throughout usually indicate a further upstream leak where air has time to mix with the water column.
Close the main drain valve (if separately valved) and see if the pump holds prime on skimmer suction only. Then open main drain and close skimmer. Whichever line causes prime loss is where the leak is. Narrow the search to one line at a time.
For suspected underground leaks: cap or plug the suction port at the pump, pressurize the suction line to 20 PSI with a pressure gauge attached, and watch for pressure drop over 15 minutes. A drop indicates a leak in the buried section. This requires a test plug or specialized equipment.
Document your leak detection findings in PoolLens. If you suspect an underground section but can't confirm it on this visit, the notes ensure the follow-up tech knows exactly where to look and what's already been ruled out.
Replace the O-ring. Clean the groove of any debris or old lubricant. Apply fresh Teflon-based lubricant. Cost: $3–8. Do this proactively every 1–2 seasons on all accounts.
Disassemble the union, replace the O-ring, lubricate, and reassemble. If the union body is cracked, replace the entire union ($8–15 each).
If the check valve flapper is not seating properly or the body is cracked, replace the entire check valve ($15–30). A check valve that fails open causes the pump to lose prime every time it shuts off — water drains back toward the pool, leaving the suction line air-filled.
A fitting that has pulled away from its joint can sometimes be re-glued without cutting if the pull-out is minor. For any significant gap, cut out the bad section and repipe with new fittings and PVC cement. Always use primer and wait full cure time before pressurizing.
Underground leaks require excavation and plumbing repair. Before digging, use an acoustic leak detection device to narrow the location. Most reputable pool leak detection services charge $150–300 for the diagnostic, which is money well spent before renting an excavator.
PoolLens lets you log air leak locations, repairs made, and follow-up checks — so no diagnosis ever starts from zero on a return visit.
Open PoolLens Free →With the pump running, coat all suction-side fittings, unions, and connections with shaving cream or soapy water. Bubbles being drawn inward identify the leak point.
Air in the pump shows as bubbles visible through the clear pump lid, a basket that is only partially filled with water, a gurgling sound, or bubbles returning through the pool jets.
Yes. A check valve with a cracked body or a failed flapper allows water to drain back toward the pool when the pump stops — creating an air column that must be re-primed every startup.
Minor repairs like O-ring or union replacement cost $20–80 in parts. A plumbing repair requiring PVC cuts and new fittings typically runs $100–300 in parts and labor depending on access.