Assuming you're not losing fluid as it appears, there must be air in the system, or the rear drum brake shoes are moving too far. For that to occur suddenly, it's more likely one of the linings has rusted off the shoe frame and has rotated with the drum away from the shoe.
There could also be a leak, often a rear wheel cylinder, that doesn't show up right away. That can be misleading because it can take a long time for the fluid level to go down. That leak can also let air be drawn in leading to a soft pedal.
Even though you didn't mention it, a loose front wheel bearing will let the brake rotor wobble and push the piston back into the caliper. You will have to push the brake pedal further than normal to push the piston back out before there will be resistance. The clue is you will be able to pump it up again and get a solid pedal when the truck is standing still. While driving is when that will show up.
You might try prying the front pistons all the way back into the calipers to push the brake fluid up and wash any remaining air bubbles into the reservoir. Normally I would tell you to never press the brake pedal more than half way to the floor when pumping the pistons back out, but the reason for that is crud and corrosion can build up in the bottom halves of the bores. Pressing the pedal all the way down runs the lip seals over that stuff and can rip them. That is not the case with your new master cylinder. That's why I asked if it was a new or used one.
Even if something is broken or there is air in one of the hydraulic circuits, half of the brakes should still work. If you apply the brakes while moving slowly on dirt or sand, see if the front or the rear brakes attempt to skid. If neither one does, look at the push rod in the front of the booster. If the push rod is adjustable, measure how far it sticks out when at rest, and how far it has to reach into the master cylinder to contact the piston.
If either the front or rear wheels lock, even just momentarily, that circuit is working, and the other one is not building pressure. That can help narrow it down to the front, rear, or something in common.
Thursday, April 21st, 2011 AT 2:49 AM