Dying with a DI

[Bluemule]

So this 02 GTX DI I have is driving me nuts! Fuel pres. issue so I replace the pump, starts but runs rough. Check the air pump and only 40 psi so I take the pump apart and the end gap on the rings are lined up! Sleeve ,rings and piston all look fairly new so I spin the rings 180 from each other, clean the head a bit and put it together. Still 40 psi. Hookup air line and run 80psi through it and it runs like a champ! Fuel pres. reads 105 and things are good! My question is do I put a new head on it, do they typically go bad for some reason or am I missing a step? Previous owner had a mechanic work on it and I am believing he rebuilt the pump. I did notice the air hose from the pump to the injection rail doesn't let a lot of air through. Normal?
Thanks

[Myself]

There is a little disc valve bolted in the head. That valve can be warped or have a piece of trash holding the valve open. Check it out.

[wjs1820]

I'm no expert but I have a 2001 RX-DI (Original owner). I ran the candoo reader after rebuilding the fuel pump ( it was sitting for 5 years ) and had to reset the TPS (Throttle position). It now runs good but not great. My top speed is down to 55 from 68. I know my fuel pump is tired but I'm ok with it. I'm going to run the pump till it dies. This may not be your issue but something worth checking.