There are a few options available for the home brewer when it comes to kegs. Home brewing kegs come in a variety of sizes, can have different post designs, and may oftentimes be purchased either new or used. If you purchase a used keg that has not been refurbished, or if your previously new keg is getting a little older, it may be time to replace its seals.
Kegs use a series of silicone seals, o-rings, and poppets in order to keep the keg both air and liquid tight. These seals are important, as they insure that your beer does not oxidize, lose carbonation, allow external bacteria in, or shower you in beer when you pressurize the keg. Time, temperature, and wear will degrade these seals, and it is wise to replace them before they fail. Most home brew suppliers will sell keg seal replacement kits, and the process is relatively inexpensive and easy to perform. There is a poppet and o-ring in each post, so you will need to remove the post in order to replace the seals. There is also an o-ring at the top of each post that creates a seal with the CO2 tank connections. Lastly, you will want to replace the seal for the keg lid. All you really need is a wrench and a pair of pliers in order to perform the replacement, and it takes just a few minutes to do.
You can find a low priced, but quality cornelius keg seal replacement kit here:
Cornelius Keg Seal Replacement Kit