The presence of the check valve suggests that someone had (or expected) priming problems before. As a first step, I'd recommend the following:
1. Get the running normally on the main drain and vacuum lines.
2. Turn the pump off and *quickly* close the main drain and vac line valves. (Hopefully, these valves don't leak.)
3. Open the skimmer line, and open the pump strainer basket lid.
4. Use a hose or bucket to fill the pump basket.
5. If the check valve is working, you will be able to fill the basket, and the skimmer line up to the skimmer. If you cannot do so, replace the check valve. (Depending on the valve, the easiest way may be to buy a new identical valve, and swap out the new valve guts for the old.)
6. Once the check valve is working, and the skimmer line is full, close the skimmer line valve and the pump basket and restart the pump.
7. Once the pump is running fully, open the skimmer valve slightly, until bubbles appear.
8. Close the valve, and leave it closed till the pump is again running normally.
9. Repeat, up to 6x.
10. If the skimmer line every runs bubble free, open the skimmer valve completely, and you should be good to go.
11. If the skimmer line never runs bubble free OR if bubbles gradually build up over the next hour of pump operation, you probably have a suction leak (letting air IN) somewhere in the piping between the skimmer and the pump.
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