Most Corny Kegs come with a safety relief valve in the lid that guards against over pressurization of your keg.As a general rule Pepsi style kegs have a manual style relief valve that allows the homebrewer to manually relieve the pressure in the keg by pulling up on the ring that is attached to the valve.
Coke style kegs have what is referred to as a “static relief” valve or “tab relief” valve which allows the keg to relieve pressure if over pressurized but cannot be manually actuated and does not have a ring style valve. If you have a lid like this you can relieve the pressure in the keg by depressing the gas fitting poppet with a small screwdriver.
You can purchase just the lid with a manual relief valve that will work on most kegs: Click Here
The short video below explains the difference between the relief valves.