If you're getting a down-the-barrel leak, the first thing to do is to disassemble the gun and remove the valve, cleaning the cupseal and valvebody thoroughly. A small piece of grit or even a fragment of hair or clothing fiber stuck between the valve and cupseal can cause a slow leak. Also check the two o-rings around the outside of the valvebody that seal it into the body. Usually a leak from these won't sound like it's coming down the barrel, but with the front ring it's a possibility.
If that doesn't work, sometimes you can rehabilitate a cupseal by smoothing the bottom face of it out with very fine grit sandpaper, but this is only possible if you can detach the plastic part of the cupseal from the metal valvestem and reattach them when you're done. Often these two parts are threaded but in some cases they're press-fit, so once you separate them they're both ruined, unless you get lucky with superglue.
I'd contact JT first, but this guy on eBay is selling Cybrid repair kits that include a replacement cupseal: http://www.ebay.com/itm/JT-Brass-Eag...-/380458158981
With as many rental guns as you have to keep working you could probably stand to have a few of these kits on hand anyway.