If you crack open the banjo fitting on the pump, it will drain the reservoir as well as the two hoses that drop down plus the solid plastic pipe that goes behind the grille. I don't know how much will drain out of the gearbox itself.

When I changed my P/S hose pair, I recall putting in about 500 ml or so back in. Don't know the total capacity of the system.

Don't forget to replace the two crush washers that are found on each banjo fitting.

BTW, are you certain the reservoir is leaking? I had an opportunity to look at mine closely (took it out of the car) when I changed the hoses and thought it was a very solid affair, including the hose barbs. The hoses themselves usually leak but it often looks like the reservoir is the culprit, when it isn't.

Regards,
Adnan