I have just installed a system in my new van and used 15m push fit plastic pipe and fittings...
The pump would not prime at all...so I thought it was a dodgy pump...sent it back and fitted the replacement...same again!
I was sat at bcak of van scratching my head and thought "I have never seen plastic tubing used as plumbing for a wfp system so maybe that could be the reason"
A quick change over to 1/2" hose and clamps and worked first time...
Yesterday, was working and the pressure dropped at brush head so I checked the water level in tank (400l flat) and there looked to be a couple of inches so carried on running the pump...looked inside the tank and seen that the level was below the outet valve so I had been pumping air for 5 minutes...
Ended up driving home to run water through the system from my garden hose pipe to remove air and then pump started up again no problems.
So don`t pump fresh air...keep an eye on your water levels.