Checking out a JD LT133 with a 13 hp Kohler CV13S. It was getting gas in the crankcase. I suspected the mechanical fuel pump. The diaphragm had a tear in it so I assumed this was the issue. I replaced the fuel pump and ran it a few minutes but it was still getting gas in the crankcase. I went to remove the carb to check the needle valve. When I removed the fuel line gas shot out of the fuel line like a fire hose. I quickly put the fuel line back on the carb and then removed the gas cap and a crazy amount of pressure was released from the tank. I cleaned the carb and cleared the hole in the gas cap. I tested it and it ran fine for about 10 minutes and started to die, I could keep it going for several more seconds by choking it. What caused the pressure buildup and could that have damaged the new fuel pump, causing the engine stop issues now?