Hi John

I suggest beginning with the assumption that your collector’s seals are all still fine and intact, so any heating engineer or plumber will be able to re-pressurise up to the 3 bar standard in these type of systems. Maybe wise to drain the Glycol and put some fresh in through the pump station access nipples.

If it fails to pressurise, then a new pump station would be required. Evacuated solar tubes are pretty basic in operation, so may have just lost pressure over time. Modern pump stations have pressure gauges attached, if yours doesn’t maybe renew the pump station as a whole.

Hope this helps.