Uneven brake pressure

Chances are the brake pedal got pushed all the way to the floor, either from the surprise of the sudden leaks, or by improper pedal-bleeding with a helper. Once a master cylinder is more than about a year old, anything that causes the brake pedal to go more than about half way to the floor can damage it. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Running the pedal over half way runs the rubber lip seals over that crud and can rip them. This is common enough that many shops automatically include a new master cylinder in their repair estimates when a vehicle comes in with a leaking steel line or rubber flex hose.

You may be able to verify this if your brake fluid reservoir has two caps on it. Remove both caps, have a helper give a quick, hard jab to the brake pedal, then watch for a spurt of brake fluid shooting up from each opening. If one of those spurts is missing, suspect the master cylinder is defective.

I'm afraid to ask this, but is there any chance a petroleum product got dumped into the brake fluid? Problems would take up to a week to show up. This includes engine oil, transmission fluid, power steering fluid, axle grease, and even penetrating oil used on fittings.

Next, how does the brake pedal feel? If it is unusually high and hard, you could have one or both front rubber flex hoses constricted and blocking fluid flow. I can't remember the hose design your truck uses, but if there's a metal bracket crimped around the center of the hose, rust an build up in that crimp and squeeze the hose closed. The more common symptom is you can apply that brake if you push hard enough on the brake pedal, but then that brake won't release. If you don't push real hard on the pedal, that brake won't apply.

Also be aware that most trucks and minivans can have a real wide variety of loading, between passengers and what you're hauling. As such, they use a height-sensing proportioning valve at the rear axle to adjust the amount of braking power to the rear. If the vehicle is jacked up or on a hoist with the rear axle hanging down, that looks to that valve as though the truck is lightly loaded, so most of the brake fluid pressure to the rear will be blocked. You may not even have a problem once the truck is set on the ground.

That still sounds like the seals in the master cylinder are leaking, but they shouldn't be with a rebuilt unit.

The next place to look is to inspect the rear brakes. Specifically, look for a parking brake cable that is rusted in the partially-applied position. That will stop the self adjusters from working. Also look under the shoes at the six "lands" on each backing plate. Those are the raised spots the shoes ride on. If grooves have formed in them, the shoe can not always retract, or it can stick when the brakes are applied.

To tell if a parking brake cable is applied, first look at the large anchor pin at the top of the backing plate. Both shoes must be resting on that pin. If either one is held away, the parking brake cable is doing it. Next, look at the parking brake strut bar between the middle of the two shoes. You should be able to push that forward against anti-rattle spring pressure, with your thumb, about 1/8". If that bar is tight, it's also because the parking brake cable is holding it that way.