I'm making an assumption you have the standard Shruflo pump like what was in the 299?
Odds are good there is a piece of dirt lodged in the internal Shurflo check valve. Taking it apart and cleaning "might" work. And might not if a scratch in the seat exists.
They do sell an entire diaphragm/check valve rebuild kit. See here for the right part numbers and then hunt on line for best price etc
Pump Repair Kits - SHURflo
Look here too under spec/parts for the model you have
SHURfloŽ Pumps - SHURflo
Odds are good you have no inlet strainer. Sunline did not put one on either the T2499 or the T310SR. I solved that problem when I upgraded the pump to have an inlet strainer. Shurflo sells one that will screw right on the inlet to the pump even and it is recommended in their pump install instructions. Or at least the older instructions. If you going in for a rebuild I high recommend installing the inlet strainer. It can save you the grief you are in right now.
They use you have a slightly different strainer then the one I can't find it right now. Maybe the stopped making it and it is now this one. This one you can take apart and clean at leasst. The older one you could not which this one is better then
Water System Accessories - SHURflo
You talked about putting a check valve in line to stop this. Well yes and no on that thought. Will it stop city water pressure forcing it backwards through the pump into the tank? = yes.
If you use the onboard tank and use the pump will it stop water from bleeding back thru the pump and loosing pump prime? Good question. If the leak gets big enough you will be dry locking the pump, loosing prime and pulling your hair out trying to reprime it. Ask me how I know this..... The internal check creates the right pump suction, prime and keep prime feature. And if you are bleeding back then the pump will cycle over time with no use in the camper when the leak get's big enough as it is bleed off pressure back to the tank.
Dirt of any size or shape is the enemy of these pumps.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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