It is an interesting delema you have brought up. I would not think that draining your brakes by gravity is a good way to flush the fluid out of your brake system. I would think some old fluid would stay in the passages of the master cylinder.

As far as Volvo's are concerned, I am sure there is a risk in damaging the master cylinder on any high mileage car that has not been regularly flushed of fluid. However, I do not know how you would change pads without bleeding the brakes a little bit after installing the new pads. Thus, whenever new pads are installed the pedal would travel throughout its range of motion. Knowing how quickly Volvos go through pads, I would think that would prevent the bottom of the master cylinder from prematurely corroding.

Another issue that comes to mind is your reasoning as to why your master cylinder failed. Maybe some of the crud that got pushed into the master damaged some of the seals? That is another reason to open the bleed valve on the slave cylinder when pushing on it. On the other hand, I agree with the other poster that I fear pinching the brake line because it may crack.