My tube / pump system is as follows:
Astral Sprint 102612T pump
9 meter x 40 mm suction hose
3 m 40 mm PVC pipe from skimmer to the pump.
The pump is slightly above the water.
When I put the vacuum cleaner hose to the skimmer, stops the pump immediately to suct and it looks as if air is trapped in the pump filter tank.
The first one is to look for leaks on the suction side. I have greased Targit glue on all joints and shift gasket at the pump and a union without having any changes. I installed a plastic bag around the entire pipe on the suction side and closed ends with tape. The theory is that if there is a leak somewhere on the suction side, the bag would be sucked empty of air. That has not happened, although the system has been running a full day of tubing. It should indicate that there is no leakage on the suction side.
The pump has 50 mm unions and on the suction side tube is pinched down to 40 mm. Between the pump and 6-way valve, there is 50 mm PVC pipe. Can you imagine that the pump capacity is so large that the amount of water that can pass through a total of 12 meters 40 mm pipe and tubing is so small that the pump is quite simply sucks prefilter tank almost empty.
When running without suction hose, everything works fine.