If your reservoir went empty, you have introduced air into the master cylinder. Most effective way to fix that is to "bench bleed" it. Just because it says bench doesn't mean you can't do it on the truck. You just loop the out lines back into the reservoir.
This is not always practical on the truck though. You can keep pumping and try to push the air out of the calipers/slaves, but that is going to take forever. Literally, forever.
I would suggest you see if you can rent or borrow a reverse bleeder/pressure bleeder.
These connect to the bleed nipples on the calipers/slaves and push the fluid up to the reservoir. The big plus to these units is they push the air in the direction it wants to go (IE: up) and it's a constant push, not a pulse like pumping brakes. You have to keep draining the master so have something handy to suck the fluid out.
Your abs unit is going to be a treat if you have air in there. You can either buy/borrow a scan tool to cycle it, pay a shop to do it or get the system bled as best you can and go lock it up a couple times on a gravel road, then go home and bleed it again. Repeat gravel road as necessary.
Brake fluid eats paint, so be careful when handing it and bleeding. It will "squirt" at the exact wrong time. Have a bucket with soapy water and a rag handy if you're nervous about it.
If your fluid is milky/cloudy, you need to flush it all out anyways. It's past it's service life and is likely water contaminated. Brake fluid sucks water right out of the air. That's why you only use fluid from sealed containers.
Lastly: if the fluid level dropped, it went somewhere. This means you have a leak. The brake system is a "sealed" system and if the level is going down fast, it's leaking. The level will drop as the pads and shoes wear and the fluid takes up the resulting extra travel in the pistons, but a rapid drop is a leak. Usually culprits of a phantom leak is rear wheel cylinders (IE: the slaves). They leak into the drum, it combines with the lining dust and becomes a paste that stays in the drum. So it leaks, but you don't see it until you pop the drum off. Unless it's an "aggressive" leak that is. Then it runs down the backing plate and drips onto the wheel where it gets flung out over the tire surface.
You can stomp the pedal and crack the lines, but you are going to be doing that for days (and probably weeks after that) to get it all done....
Good luck.
