This is a bit different than what I've read before.
This happens usually only after I start the Truck and travel for a few hundred feet.
I apply the brakes to stop, and just before it stops I hear a sound from the front, hiss? ABS? and the peddle goes to the floor. I do stop, just fine, and it usually never happens again, until the next time I start the ranger.
This doesn't happen in reverse.

I replaced the front pads, and fixed a broken adjuster in the back a month or two ago. I bled the brakes twice since then. I wonder if air is getting into the system? The peddle is a bit spongy, and bleeding them improves that.

No fluid loss, but it is very green looking.
I tested the vaccum booster and it passes the tests as far as I can tell, with the exception that the peddle drops much more than a 1/4 inch when I turn off the truck with the brakes applied.

I've also tested the vaccum line and the check valve.

I am thinking about replacing the calipers just because they are original, along with brake lines which are pretty rusty. I'd just like some speculation or things I could try to narrow the problem down.

 

First, ABS prevents brake pressure from going to a wheel, it doesn't use brake pressure
If ABS is activated you won't be able to push down on the pedal, you will feel a pulsing in the pedal as it STOPS brake fluid from going to a wheel.

Pedal going to the floor reads like a leak, if there is no visible leak then it could be in the Master
Master has three seals, O-Rings, the piston inside has two chambers and then a seal on the end
Image here: http://www.aa1car.com/library/brake_master_cylinder.gif

If one seal was leaking then pedal could go to the floor as brake fluid flowed from one chamber to the other instead of sending fluid out the brake line to the Slaves/calipers

 

Finally got back to the brakes on the ranger. I replaced the master cylinder, and the Vaccum booster. With still the same symptoms. When I stop, right before I stop, I feel the brake peddle give, and hear a sound like air escaping. I can hold the brakes when I am already stopped and I don't loose peddle pressure. This only happens when I am moving, and not every time.

 

The atmosphere valve is at the cab end of the booster, when you press on the brake pedal it pushes open that valve and air rushes into the booster which gives you the "power" in power brakes.

If you have cruise control there is another valve on the brake pedal, a Vacuum Release valve.
The Cruise control actuator uses engine vacuum to hold throttle open when activated.
The brake pedal vacuum release valve removes/releases vacuum from cruise actuator so it can no longer hold throttle open, it is a safety feature.

If you got a new booster then it would have a new atmosphere valve, new doesn't mean it is OK.

If you have cruise then locate the Cruise actuator in the engine bay, remove its vacuum hose and PLUG that hose with a screw or bolt, or tape or ??, so no vacuum leak

See if the noise goes away, if so then you will need to replace the vacuum release valve, or live with the noise, usually there is a filter that muffles the noise, but there is always some air noise in the cab with power assist brakes, it just shouldn't be that noticeable unless you are listening for it


Between the booster and master there is an adjustable push rod, it needs to be flush against the master piston when installed, so no gap, or you will get what you described, pressure when you push on the pedal then a gap and pressure again as push rod finally pushes on masters piston.

Or you could need to bleed the brakes, if there is some air in a line then it compresses when brake pedal it pressed then when that air is fully compressed you feel full pressure of fluid

 

What year truck do you have?

How many miles on the truck?

Is there any warning lights on the dash brake or ABS check engine ?

When you replaced the Master cylinder did you bleed all four comers?

Did you bench bleed the master cylinder?

Does your master cylinder have a electronic valve mounted to it just in front the reservoir and a bleeder on the master cylinder?

If it does I think it is the cruise control deactivation switch.

( I ask because on my truck I had to bench bleed my master cylinder and bleed at that switch because a slight amount of air was trapped under the switch ).

Did you have any issues getting brake fluid out of any of the calipers or wheel cylinders while bleeding them?

How do the brake flex hoses look like? Do you see any dry rot or other signs that they should be replaced?

Have you inspected the brake pad slides and caliper slides?

Does the truck pull or vibrate when applying the brakes?

Does your parking brake hold properly?

How far down does the parking brake go with normal pressure from your foot applied?

Have you double checked the rear brake shoe adjustments?

If the rear brakes are adjusted properly and there is no air in the system, you should be able to pump the brake pedal with the truck off and get a full hard pedal that will not bleed down.

Once you start the truck the pedal will drop some.

Does your truck behave this way?

This is not brake related but have you inspected all the front end and suspension parts for play ?

Could your problem be play in the front end that is causing movement and is felt through the brake pedal?

It sounds like to me that you have or had more than one problem.