What difference is there whether the master cylinder clamped in a vise or bolted on a vehicle? Answer, NONE. As long as you get the air out, you're good. I flat rated for a few years and as long as the m-cylinder was level on the car or truck, I bench bled them on the vehicle. I did crack the line fitting at the last moment to keep any air from getting into the brake lines. I always felt that as long as the original brake lines were still full of fluid, there was no air gonna jump in there, so no need to bleed them out. You may want to rebleed the lines, but that is your choice, and not a necessity.