Master cylinder removal

You don't HAVE to remove the brake fluid first. That is just done so not so much spills out and eats the paint off the truck. One tip that will make the job easier is to just loosen the steel lines, unbolt the master cylinder, then use it as a lever to bend the lines upward just a little. That will prevent the fluid from running out when you unscrew the lines.

Bench-bleed the new master cylinder, screw the lines onto it, push it down to bend the lines back to normal, bolt it in place, then have a helper slowly push the brake pedal halfway to the floor. As he does, air bubbles will come out of the lines that are still loose. Tighten the nuts, then allow your helper to release the pedal. Do that a second or third time, then slowly press the brake pedal while the steel lines are still tight, then let the pedal return rapidly. Any tiny air bubbles that are still in the lines will be washed up into the reservoir along with the returning fluid. By doing it this way I rarely have to bleed at the wheels.

For an old master cylinder, never push the brake pedal more than halfway to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pressing the pedal to the floor, as many people believe you must do when bleeding the brakes, runs the lip seals over that crud and can rip them. That is not a concern with a new or freshly rebuilt master cylinder.