Flat beer in a keg is usually due to the keg being under gassed or the keg having a gas leak. Before you gas the keg a second time you will need to check the keg for leaks. To do this you need to set your regulator to 40 psi (280kpa) and pump gas into the keg for about 5 minutes. This will be enough to test the keg. Once the keg is full of gas get a spray bottle with a mixture of dishwashing detergent and water and spray the mixture onto the Hatch Cover, Gas Post and Liquid Post, lines and fittings. Don’t be shy with the detergent and make sure the whole area is covered with the solution (detergent and water).
If bubbles appear you will be able to see the leak and need to repair the keg. Usually it’s the Popette valves in the gas and liquid posts or it’s the Hatch Cover Seal. All of these parts are available from us online. If no bubbles appear around the Hatch Cover, Gas Post or Liquid Post you should spray detergent all over the keg especially around areas where the rubber head or toe of the keg joins the stainless body of the tank. These areas are pron to rusting under the rubber and getting pin hole leaks.
If no bubbles appear it is safe to say the keg is OK and you should proceed to re-gas the keg. When gassing a keg the easiest method is to attach your gas line, turn it to 40psi (280kpa) and allow it to sit under pressure for 36-48hrs.