Water pressure is being lost. Water is leaking back through the pump diaphram either because there is something in there or it has a hole.
Do you have a filter before the pump?
Cheers,
Peter
Look very closely for weeps or drips under the van, in all cupboards and tap connections. It only takes a slight weep to lose pump pressure momentarily.
Usual cause is John Guest connector or split in JG tubing.
Every 3-4 minutes indicates a weep.
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Cheers, Richard (Dick0)
"Home is where the Den is parked, Designer Orchid Special 21' towed by Jeep Diesel Grand Cherokee Limited"
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You have a leak downstream of the pump. I had this exact problem for several years and could not find the leak, no matter how hard I tried. Only recently did I finally discover that it was a slow tap leak, but so slow that it took many hours for the drips to become apparent. If you can't find a leak, then replace the tap washers.