Could use some tips on debugging Lake Water pump

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I'm new to this property that I'm dealing with on this pump. I noticed the pump was running a lot a month ago when we viewed it. It's not holding the back pressure well enough. But it was getting by, just running a lot.

I did shut it off about a week ago to do some electrical work at the house. I had a heck of time getting it going again. Had to prime several times and it sucked in (or stirred up) a lot of sediment. The sediment clogged the toilet valve which I had to replace. It was back to getting by.

I just wanted another month out of it (if weather cooperates). I was going to add a backflow valve near the pump but was worried that might make it hard to prime. I also bought a new pickup for the end of the line that has a backflow valve in it.

I was gonna work on it and decided to let it go. Sure enough, I think the back pressure leak got worse and it lost it's prime. I'm hoping/assuming the pump is still ok. If not, it's time to close. It was free running for ~24 hours. Not sure how much water was still in it, but it was not building pressure.

The main hesitation I had working on it, is working with that stiff plastic line when it's cold out.

How would you find the back pressure leak?

I was thinking of just replacing the pickup valve and cross my fingers. Pickup is only in about 1 foot of water right now. Lake is down about a foot.

Should I just cut it off and whack on a new one?
Do I need to heat the tubing to make it pliable?

Should I put the backflow valve in near the pump?

Any other ideas appreciated.