Pump loses prime right after shut off

[michael734]

Hi

Our pump has been losing prime right after shut off for a good few months now and I can't seem to figure out the cause. When the pump switches off, it takes just a few seconds for the water to go all the way down. Before the water goes down, I can hear what sounds like water blasting/bubbles inside the MPV. I've already changed the O-ring on the MPV but it didn't make a difference. I can't spot any leak in the pipes either. Does anyone know what might be causing this?

I've also tried using a hose to find possible cracks in the above ground plumbing. The pump basket lid is also tight and the pump doesn't have drain plugs.

Any thought appreciated.

[Teapot]

Can you block the waste pipe with an air tight bung? When you say O ring on the MPV, do you mean the spider gasket?
The pump pre filter lid gasket is another very common place to look. I assume the pump is above the pool water line, if so there should be a non return valve before the pump, they sometimes get blocked and won't close

[michael734]

Can you block the waste pipe with an air tight bung?

I've tried blocking it with plasticine but it still loses prime.

When you say O ring on the MPV, do you mean the spider gasket?

I mean the O-ring seal on the bottom of the MPV cover. I have indeed replaced the spider gasket as well.

The pump pre filter lid gasket is another very common place to look. I assume the pump is above the pool water line, if so there should be a non return valve before the pump, they sometimes get blocked and won't close

There's no pre-filter in my setup. The pump lid O-ring looks quite springy though. How do you tell if it needs replacing? Yes, the pump is above the pool water line. There's no valve before or after the pump though.

[Teapot]

Ok, did this issue exist before you changed the spider gasket?
The pump lid, is the pre filter, although the seal is still spongy, which is good a bit of silicon grease may help form an air tight seal.
No non return valve, you may need to look at adding one.

[michael734]

Ok, did this issue exist before you changed the spider gasket?
The pump lid, is the pre filter, although the seal is still spongy, which is good a bit of silicon grease may help form an air tight seal.
No non return valve, you may need to look at adding one.

Apologies for the late reply. No, the issue did not exist before the spider gasket was changed. It had been fine for 3 months actually. We also had a pool shop take apart the MPV cover and the handle. They replaced the washer and O-ring. I've already added silicone grease to the lint cover's O-ring.

When I shut off the pump, I hear gurgling and see leaves under the lint cover get sucked back into the suction line. Then the water under the cover lowers to about just 2cm. I'm guessing I might need to trap the air inside the pump and filter in order to stop losing prime. Do check valves usually lock air in?

Thanks