This may be caused by an air leak on the intake side. Perhaps a connection was disturbed when the petcock was removed and installed? When this happened on my trailer, I found a split in the inlet pipe. You can establish that the pump is OK by detaching the inlet line from the tank at the pump, attaching a short length of hose to the pump inlet, and placing the other end of the hose in a bucket of water. Opening a faucet should then pump water out of the bucket. Strictly, you would want the level of the water in the bucket to be equal or lower than the water in the tank, but I didn't bother. Assuming the pump is proved OK, look for loose inlet hose clips or a split hose. If there is no apparent problem, and the tank is indeed full
, then check that the breather pipe is not blocked. On my trailer, inside the water flap is a small opening, the open end of the breather pipe. Using a suitably sized straw you can blow down this to ensure the breather pipe is clear. On my trailer there is a small plastic plug on a short plastic strap which is used to block the breather during storage, to prevent insect ingress. One year I forgot to remove the plug, and the symptoms were as you describe. My best guess is that you have a loose clip at one end of that inlet pipe. Good luck.