There are not many systems involved with the APC system. The Bentley has an excellent troubleshooting section in it... I can't imagine they'd leave something out. When I troubleshoot an APC system, it goes something like this:
1. Check base boost. Unplug the APC solenoid, plug in a calibrated boost gauge, and take measurements.
2. Check to be sure the APC solenoid is being energized with the key on. Connect a voltmeter to the APC solenoid leads, and turn the ignition to Run. You should get battery voltage here. If you don't, you have a problem with the APC electrical system (not vacuum). Plug it back in and proceed:
3. Check to be sure the APC box is plugged in, and that it works. They rarely fail, but when they do it's always the last thing you check... Ask me how I know... :)
4. Check to be sure the cruise control vacuum switch on the driver's side fender is working properly. I don't know if this is present on 8v cars, but I suspect it is.
5. Verify that the pressure transducer under the dash is working correctly. If it's not, you will end up with base boost. Try unplugging the vacuum line to it (cap the vacuum line) and see if that fixes the problem. If it doesn't, jumper the two electrical leads and see if that does. NOTE: In both of these tests, the APC system will not know how much boost you're making and will NOT regulate it... Be light on the accelerator - just see if you get more boost or not, then back to the garage.
6. Test the cruise control pressure switches above the brake and clutch pedals. It doesn't matter if they hold vacuum or not, but it does matter that they have continuity when the pedals are in the rest position. If you don't have continuity, that's your problem.
If none of these solve your problem, you may have a vacuum problem. Check to be sure you've got no intake leaks, and that all your vacuum hoses are in good shape. On an 8v car, the difference between full boost and base boost is only a few psi, and vacuum leaks can have bad effects. At the intake manifold, pull off each vacuum line one by one and test them... be sure you test for pressure and vacuum. If that yields nothing, pull off the intake plumbing rubber boots at the turbo and the throttlebody and verify they are not cracked or broken.
If *that* yields nothing, try this: Disconnect the 'w' port on the APC solenoid and drive around. At this point, you should get wild, uncontrollable boost. Be VERY cautious when driving like this, and only drive long enough to confirm you're making lots of boost. You can nuke your engine driving like this for too long. Take it out, floor it, and see what happens. Then back to the garage. If you don't get lots of boost, your turbo has a problem. If you do get lots of boost, you've got more issues than I can help with... ;)