Hayward MaxFlo VS 500: Pump Won't Prime or Loses Prime

How the MaxFlo VS Primes

The MaxFlo VS 500 is a self-priming pump but it needs help. Priming requires that the strainer housing is pre-filled with water, all suction and discharge valves are open, and there are no air leaks anywhere on the suction side of the system. When you power the pump on after filling the strainer, the drive starts the motor at MAX allowed speed for the configured prime duration (default 8 minutes). If the pump does not develop suction within that window, it stops and you need to troubleshoot why.

Key rule from the manual: the pump should NEVER run without water. Water cools and lubricates the shaft seal. Running dry—even briefly—will damage the seal and voids the warranty.

Step-by-Step Troubleshooting

Owner-Level Checks

1. Pre-fill the strainer housing

• Remove the strainer cover (SPX2300DLS). Verify the o-ring (SPX2300Z4) is seated in the groove, undamaged, and lubricated with Jack's 327. A dry or cracked o-ring will not seal.
• Fill the strainer housing completely with water until it reaches the top of the housing or the suction pipe level.
• Replace the strainer cover and tighten the lock ring by hand only—no wrenches. Over-tightening will crack the lid.

2. Check pool water level

• Pool water must be at least halfway up the skimmer opening for the skimmer to maintain flow without sucking air.
• Low water level is one of the most common causes of air ingestion and prime loss, especially in hot weather with evaporation.

3. Verify all valves are open

• All suction and discharge valves must be fully open before starting the pump. A closed valve will completely prevent priming and can cause dangerous pressure buildup.
• The manual is explicit: "All suction and discharge valves MUST be OPEN when starting the circulation system."
• If the system has a three-way valve selecting between skimmer and main drain, confirm it is set to allow flow.

4. Watch for bubbles at pool return fittings

• Start the pump and observe the pool return fittings. Steady bubbling from the returns confirms an air leak on the suction side of the system.
• Common air leak points: strainer lid o-ring, union connections at the pump suction, PVC glue joints that have dried out, valve packing glands, and cracked pipe.

5. Clean skimmer and strainer baskets

• A full skimmer basket blocks flow from the skimmer, forcing the pump to pull harder and draw air through any small gap.
• A full pump strainer basket (SPX2300M) does the same thing. Always check both before concluding there is a bigger issue.
• After cleaning the pump basket, refill the strainer housing with water and tighten the cover before restarting.

Tech-Level Checks

6. Locate and fix suction-side air leaks

• With the pump running, systematically check every union and threaded fitting on the suction side. A small amount of water poured over a joint while the pump runs can reveal air ingestion by a change in pump sound or by water being pulled into the fitting.
• Loose union connections: hand-tighten the union nuts. Do not use wrenches on plastic unions—you will crack them.
• Threaded connections: if leaking, shut down, depressurize, remove the fitting, clean off old PTFE tape, and re-wrap with 1–2 layers of fresh PTFE tape before reinstalling. Do not use plumber's pipe dope—it can crack plastic.
• Valve packing glands: tighten or replace the packing if the valve stem is the air source.

7. Check suction vacuum with a vacuum gauge

• The manual states you should have 5–6 inches of vacuum at the strainer cover with normal suction conditions. Your pool dealer can confirm this with a gauge.
• If vacuum is too low, the suction line is restricted or there is an air leak. If vacuum is normal and flow is still low, the problem may be on the discharge side (see the Low Flow article).
• A quick field test: with the skimmer basket removed and the skimmer full of water, hold a flat object over the bottom port of the skimmer. You should feel strong suction. No suction indicates a line blockage between the skimmer and the pump.

8. Inspect the impeller for partial obstruction

• A partially jammed impeller (SPX2710CM) can develop some vacuum but not enough to fully prime the system.
• If a vacuum is developing but the pump never fully primes, shut down and open the wet end to inspect the impeller for partial debris blockage, hair, or other restriction.

9. Check and adjust the prime duration setting

• Navigate to Configuration Menu (hold < and > simultaneously for several seconds to unlock) and find the Prime Duration setting.
• Factory default is 8:00 minutes. Range is 0–15 minutes in 30-second increments. If the system has a long suction run or significant vertical lift, the pump may need more time to prime. Increase to 10 or 12 minutes.
• The prime cycle runs at MAX allowed speed. Confirm MAX allowed speed is set appropriately (default 3450 rpm).

10. Check the shaft seal for leaks that feed air into the pump

• A failing shaft seal (SPX2700SAV) can allow air to enter the pump housing from the motor side, especially when the pump is below pool water level.
• Shut down and inspect the seal plate area for water seepage or mineral deposits that indicate seal weeping. If you find evidence of leakage, replace the shaft seal before trying to prime again. See the shaft seal leak article for replacement procedure.

Priming Configuration Notes

• Prime Duration is set in the Configuration Menu under item 9. The pump primes at MAX allowed speed for the configured time.
• If Low Temp Operation is enabled and triggers, the pump primes first then runs at MAX speed for 8 hours.
• After a strainer basket cleaning using the Stop/Resume button, press Stop/Resume again to resume. The pump will restart its normal timer schedule, not re-prime automatically unless the next timer cycle begins.

← Back to MaxFlo VS Troubleshooting Guide