It's not necessary to have gas valves and such for cider but to ensure a good head of co2 I use an injector for the barrels of 'draught' (as 9, 16 & 17 of 2010 remain unbottled - 14 too up to last week where the last glass or two became a little cloudy as the lees were drawn through)
As Andrew Lea points out cider doesn't lend itself to barrel/keg conditioning under pressure, be it generated naturally by fermentation or introduced, but I find it useful to help get the cider out without introducing air via the tap which then passes up through the cider and any co2 'cover'. Without a little pressure the cap can be loosened as a partial vacuum develops but again this introduces air (though it doesn't 'glug' up through the contents this way and is preferrable) and will sit above the co2 'cover' (as the heavier than air video demonstration shows here)
So that's the 'long' of it - the 'short' of it being that a flying visit was in order for a refill.
As I've been doing some re-wiring of the garage electrics I was held up in discussion at ReCon Electrical (conveniently on the same industrial complex) on the best way to get electrics to the pond equipment... another story... and so it was dark and past closing time when I eventually remembered where Hambleton Bard's new premises were...
Clearly inconvenient, as the guy there was literally just coming out of the door ready to lock up and for many it would have been too much trouble, but no, 'no problem', said he, returning a few minutes later with a full co2 cartridge.
I post this as I've just revisited the site to see if they sell direct online yet (seems not) but the bag-in-a-box machinery video kicked off, which I thought was rather interesting... for wine kits you understand not cider, but one can dream...
(to see the video click on above HB link above and scroll down a tad)