Although I mentioned it, better to not run the snake. Because air, and then water, came out of the pressure valve, we know you have at least some flow going into the filter before it stops flowing.
Have you tried having just one valve open, and the other closed (and vice versa)? That would tell if one line was bringing in lots of air.
Are you sure the check valve is installed for the correct flow direction? (sorry, obvious, but have to ask...)
Can you find fittings/pipe to take out the check valve and just run without it for testing? Nice to be able to, but sometimes those unions don't have common hardware store threads....
My pump, which seems the same as yours, would falter ( fill with air and not pump as well or at all after it had initially pumped water) commonly if:
1. I had the intake valves closed (you don't - but same effect as the backflow not allowing water to pass).
2, It sucked air from the returns (hence the questions about the skimmer, and testing with only 1 valve open).
3. Filter got extremely clogged with dirt, and it had a hard time pushing water through it (but you've already looked at that).
4. The impeller was clogged with lots of debris.
5. I put the clear cap on without the o-ring. (A bad o-ring or cap will let air in, but it is usually slower, and not the quick fail you seem to be seeing).
If I was really oblivious, and had my output valves shut, it wouldn't even prime. So I'm guessing on yours it is less likely you have an extreme blockage after the filter.
Have you tried having just one valve open, and the other closed (and vice versa)? That would tell if one line was bringing in lots of air.
Are you sure the check valve is installed for the correct flow direction? (sorry, obvious, but have to ask...)
Can you find fittings/pipe to take out the check valve and just run without it for testing? Nice to be able to, but sometimes those unions don't have common hardware store threads....
My pump, which seems the same as yours, would falter ( fill with air and not pump as well or at all after it had initially pumped water) commonly if:
1. I had the intake valves closed (you don't - but same effect as the backflow not allowing water to pass).
2, It sucked air from the returns (hence the questions about the skimmer, and testing with only 1 valve open).
3. Filter got extremely clogged with dirt, and it had a hard time pushing water through it (but you've already looked at that).
4. The impeller was clogged with lots of debris.
5. I put the clear cap on without the o-ring. (A bad o-ring or cap will let air in, but it is usually slower, and not the quick fail you seem to be seeing).
If I was really oblivious, and had my output valves shut, it wouldn't even prime. So I'm guessing on yours it is less likely you have an extreme blockage after the filter.