I did see information on this a few years ago in a magazine. It was for the larger aquaroll with the side cap and basically involved replacing the cap with a valve.
I priced up the bits to make my own last year and found it wasn't worth my while once I found this guy on fleabay selling his own version. Works perfect (after a tweak of the position of the fitting on the filler cap) and nothing in it price wise once you've faffed around.
Quote: Originally posted by alpiner on 23/5/2011
Torbeck side entry valve ... all you have to do is make a hole in your filler cap to fitt
... and get some decent food grade hose, 10m, circa £15
Hose fittings, a few extra quid
Replacement Aquaroll cap, for those times you're not on a serviced pitch, £6 ...