I own a KJW USP Tactical (and please don't just reply with "sorry") and of course I'm faced with the infamous mag leak problem and I would really like to be able to replace my valves. I've found valves to buy, but I can't find the tool itself needed to remove the old valve. I've heard of making them by dremmeling out the middle of a screw driver but I'd really rather buy the tool itself. If anyone knows where I can buy one, or any other ways of removing those valves, I'd love to hear about it.
~edit - I've fixed your slightly vague title. Check out this thread if there's any confusion. -rantng
The valve on some mags might not be recessed enough that you will need a valve tool, although a flathead screwdriver has always worked for me. A drop of silicon oil on the o-ring solves most leaks.
airsoftgi sells the valve tool, but it is pricey at $18 - http://www.airsoftgi.com/product_info.php?products_id=462 You might have better luck at homedepot or ebay. Personally, I'd rather pick up a toolset that includes a valve key. I'm sure they have other applications than solely for airsoft valves so you should be able to find one for cheaper at a general hardware store.