The AW8A can be seen at the web site below:
http://www.baker-mfg.com/domestic_new/at_the_well_control_systems/at-the-well39.pdf
However you describe that the pressure switch is and has always been under the house so I doubt that you have the AW8A.
You will need to completely disconnect the old tank because I believe it is leaking water. I doubt that the pitless adapter is leaking however if you want to look down the well, use a small hand mirror and focus the sunlight down the well. You should be able to see if it is leaking.
If the pressure continues to drop after disconnecting the old tank it could be that the pump check valve is leaking back. However I doubt it.
NOTE: It's always best to have the pressure switch mounted near the pressure tank.
I suggest that you look up
http://www.cyclestopvalves.com/pdf/psidekick-brochure.pdf it will show you how the pressure switch is near the tank.
The Cycle Stop Valve gives you constant pressure like city water pressure.
Porky Cutter, MGWC
National Ground Water Assn., Certified "Master Ground Water Contractor"