The problem with old SID rear shocks...

   If you have an older Trek STP frame with a SID rear shock, it probably looks like this inside.

   The stripes are wear marks in the strut; these are bad. If your (-) pressure constantly drops this is likely the problem. If your shock is not worn like this, that is awesome and you should put new seals in the shock while you still can.

   I see this all the time and you have very few options once your shock is bad, there is no easy replacement for the STP.