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Checked the expansion vessel, pre-charge was only 1.2 bar. Pumped it up to 3.5 bar and retested. Holding at 3 bar over 30 minutes now. Cheers Gary, would never have thought of that first.
Also double check the PRV isn't weeping. Sometimes the seat doesn't seal perfectly on new ones and they drip just enough to fail a test. Put your hand under the tundish while it's pressurised and see if anything's coming through.
Check the expansion vessel first — if the pre-charge pressure is wrong or the diaphragm has failed it'll show exactly this symptom. You should see a pre-charge on a new cylinder of around 3.5 bar (check your Megaflo datasheet). Put a tyre pressure gauge on the Schrader valve with the system depressurised and check the charge. If it reads 0 you've got a failed diaphragm.
Classic, factory set low and nobody checks before install. Megaflo should honestly ship them properly charged but here we are. Good result, worth noting in your commissioning paperwork.
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