As the heads turn the rollers pinch the dosing pump tube and this pulls the fluid around the pipework, when the heads stop there is always at least one roller pinching the pipe and this should prevent any drain back .

However if the pipes are not connected correctly to the head nipples air can get into the pipe and the fluid can drop back down the tube , equally when the heads start to wear after many months of use the dosing tube can flatten a little and this may cause fluid drain back albeit very slow drain back.

On the P1 Pro and P4 Pro dosing pumps always check that the collets have been inserted into the head junctions correctly and haven’t been reversed or crushed when tightening the collar as that will distort the pipe on the nipple and cause possible air leaks into the pipe.

If you are using different pipework also check that its the correct diameter as if its slightly loose this can also cause air to get into the junctions when the head isn’t running.

Non return valves can also be installed into the pipework, this will prevent any drain back, but if the pipe is sealed correctly at all junctions especially the junction on the ‘in’ side of the head the you should not get drain back as long as one of the rollers inside the head is pinching the tube.