This isn't likely to be too helpful but my PAS system drained all by itself - when the hose wore away against a brake pipe!
More helpfully : Manual says drain the reservoir with a syringe but nothing about the rest of the system. What I'd look at doing is to undo the hose at its join with the metal pipe directly below the brake/clutch reservoir. It's right at the base of the bulkhead. You'll see the hose that runs past the coolant header tank ( from the PAS cooler in front of the radiator ) drop down to its join with the metal pipe. This is close to being the lowest point in the system but it still wont get everything out as the metal pipe turns up to join the hose. Maybe put a very low pressure air line ( you don't want fluid spraying everywhere! ) on the hose to force the rest of the fluid through the pipe?
You'd need to remove the intake gubbins for access unless you're working from below.
The bleeding procedure on refill is fairly easy. Fill reservoir and turn the wheels lock to lock whilst topping up the reservoir as necessary.
