Isn't that what all the rookies say: "got it this time!" Well I hope I do. Time will tell again.
I first did the rebuild with only a water faucet O-ring at the last check valve. It worked one time but not the second. I was happy. I thought it was the O-ring being too soft, but now I don't believe that was the case.
I found an assortment box of O-rings at Harbor Freight made of 'Viton' a polymer designed to withstand up to 400 degrees, for $10. I put one of those Viton O-rings in place of the water faucet O-ring and there was no improvement at all. And inspection of the softer O-ring showed no damage - so the soft ones will probably work on the last check valve.
So I was convinced that I needed a complete rebuild but I didn't want to buy a complete seal kit from AirForce - still believing that one doesn't need to actually replace ALL of them.
What I ended replacing -only- and it works now,,,,very nicely is only two O-rings,,,the very last one on the cone shaped brass check valve and the piston O-ring. Turns out that the piston O-ring does all the traveling up and down the shaft and so it makes sense it would wear out, but it didn't show visual evidence of wear.
I had the pump completely apart and I believe this is important for a good cleaning - there was water drops thru out the innards.
I also used the Permatex Anti-Sieze lube thru out, designed to withstand 1600 degrees.
So,,,I used only two O-rings,,,Viton material which is harder material than the original ones,,,,and it seems to be doing great. I'll give a report after some time has passed.
Here are pics of the two O-rings I replaced
