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Self-priming automatic electric pumps with helical gears, produced by Marco.

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Automatic air drain on air vent

My pump is installed on a sailboat for fresh water. Fresh water tanks are flexible and there is often air passing into water hose. I must unscrew the air vent cap to remove air and prime the pump. I wonder if I can install an automatic air drain, like on radiators at home. I don’t find the characteristics of the air vent cap screw. Could you help me fixing this? I assume many people have this problem with flexible tanks. Thank you

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Hi @llq ,

What is the model number of the pump?

Perhaps @teof may be able to help. ;-)


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Hi @llq , can you check whether the air is coming from the tank or from something in the plumbing before the pump? a 3-way valve, faucet, connection or the filter?

The pump shouldn’t have problems priming, the air vent is there to be used only when the gears are completely dry or the pump is very old and the gears are worn out.

If you see that the issue is in the filter, unscrew it on both sides and add some teflon tape to the connections. When placing the tape, make sure that no abundance of it makes it way into the pump, because it could be an issue for the check valve (if your pump is electronic).

If the issue is in a faucet, connection or three-way valve before the pump, apply the same principle of trying to stop the air from going into the hoses. Some use the “liquid seal” for all the connections, to be sure.

Anyway, the air vent cap screw should be a metric M5 (I’m on holiday, if you need to be absolutely sure I have to check), but automating the vent it’s not a real solution, but a workaround :)

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Hello Matteo

Thank you very much for your reply.

There is no air coming from something in the plumbing or filter. When filling the flexible water tank, about 1 liter of air remains close to outlet. This is this air that I need to take out when I fill in my tanks.

My pump doesn't prime if I don't use the air vent screw.

Pipe is making a goose neck about 50cm high before the pump (because of integration of valves). It could make priming more difficult, I am going to try changing this.

Thank you again and have nice holidays


I check, I confirm the screw is M5, thank you!


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Loys wird auf ewig dankbar sein.

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