From my experience new Trane units do not have schraders under the HPS. From the thread jimj linked I was told that there is a new standard as of 2005 that states there can be no valves of any sort between the compressor or condenser (not sure which one) and the HPS. I would either put in a tee like smj recommends or braze on a saddle tap.
No cust is going to pay me to recover a unit to change out a hps. So, I will install a swivel on a high side port, mount a new sw on one and use the other for an access. Wire it, test it, done.
I just read the other day on here that OSHA made a law against that because they felt it was putting a valve on a safety device much like putting a valve on say a pressure relief valve on a boiler... Not that anyone can manually close a schrader core.
I am yet to see any proof of this.
I treat every pressure switch as if it doesn't have a core behind it. Loosen it and give it the wiggle test as you slowly unscrew it, if it doesn't stop leaking by the time you get down to the last two threads you know you are screwed.