It would make sense that the one giving you some problems is the one pump with 2.5 meters of lift.
Maximum dry lift for a PT-05 pump is 3 meters which in theory means the pump still should prime provided the fluid has a specific gravity of water.
If the specific gravity of the fluid is larger than water you then will need to lower the lift valve. (3 meters maximum of water is the same as 2.4 Meters of a fluid with a S.G. of 1.25) Also frictional pipe losses can come into play and lower the maximum amount of lift a bit. The customer also may have the smallest of vacuum leaks in the suction line or o-ring seals at the valve area of the pump and this may slightly lower the lift ability. Note that if your customer puts fluid in the suction lines and/or pump the pump will have increased suction lift capability. (Note that the wet lift of the pump is 7.9 meters of water) This explains why the pump will work when the pump has priming water in it. Also sometimes if there is a very minor vacuum leak in an o-ring seal it will stop when the o-ring is wetted.
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