Compressor Rebuild

Yes, I did get it all put back together, but haven't charged it yet.
There some reed valves in each head of the compressor,and they do rust, but they usually leak pressure, not build it.
The only thing I can think of is when you charge it, don't turn the can upside down. Doing that let's in fluid, not gas. Fluid's don't compress, and that would cause it to lock up.