I had the same problem this spring. I took the pump apart and cleaned the check valve. There was a couple of small bits of plastic in it. I put it back together and it still leaked. So I contacted Shurflo and the pump was still in warranty so they send me a new pump head. I installed it and no more problems!
If your pump is relatively easy to access the repair is very simple.
'12 Avenger 26BH
'01 Ford Excursion V10 4x4