Leaking Release Valve

Once the gas runs out, O2 will enter, unless your leak is configured in a way that leaks under medium/high pressure but seals under lower pressure, like an unintentional spunding valve. That seems unlikely to me though, especially since you say your CO2 was gone after sitting at 20 PSI for a while.

Have you tried pulling-then-releasing the valve carefully? Sometimes just reseating can temporarily solve a leaky relief valve.

 

Could it be gunked up and not allowing it to seal? Those things screw off. Take a look at it.

 

If whatever store you're going to is well stocked, you might be able to buy a new PRV without having to buy the whole lid. Come to think of it, I think it may be more likely for any given LHBS to have the PRV than whole lid assemblies.

 

^^^^ what he said ^^^^
A new lid will cost you $15.95 (overpriced, IMO), while a new PRV is only $6.95 (more overpriced than the lid, IMO, but still cheaper). Prices vary.

 

Second. I had some hop debris in my release valve and the same thing happened. It was impressive how little vegetation it took to ruin the seal.

 

If you have several kegs, replacement parts are good things to have on the shelf.