Some general ideas but not up on the way to do it?
The newer systems have a fluid reservoir that is divided. One half for front and the other for the rear wheels.
In between those two sections that we might call two tanks, there is a sensor that is supposed to be set so it is centered, not touching front or back if we can figure that? The safety idea is that when some small leak start, it will be on the front or rear and as the fluid leaks out, the sensor moves due to uneven pressure and that turns the alarm light on at the dash! It lets us know we are beginning to have a fluid leak problem and it does it before we totally lose either front or rear brakes.
We can still have two of four wheels stop us but it is not as good and solid.
But if the fluid gets too low, air will get into the brake lines from the front master cylinder to some of the brake lines and the wheels.
When you say the brakes are not as good and they feel spongy, that says something is losing fluid and there is air in the system. Since air can be squeezed, that if why it feels spongy as you get some of the pressure to the wheels but some of your pressure is used up squeezing air!
The solution is finding where fluid was leaking out and fix that as a first step. It is possible to spot it sometimes if we look under the "truck" at the inside of the wheels. One of the common places to leak is at each wheel where there is a wheel cylinder. The wheel cylinder has arms that stick out and move the brake pads to press again the wheel to stop it.
You step on the brake pedal and it moves fluid from the master cylinder to each wheel and moves things to press the brake shoes or pads to stop us!
The problem is that there are rubber type seals at each wheel to let things move but not leak. Until they get worn! if there is a leak at the wheel, it often winds up throwing brake fluid out on the inside surface of the wheel and tires. If we look under and a wheel has an oily coating on the inside, that is the major clue!
If fluid was leaking and more added, there may still be a leak but to get things fixed we have to find and fix the leak and then as part of getting it back to normal, there is some method to recenter that sensor so it is not touching front nor back.
Way back the idea was to watch the light while somebody underneath let just a tiny amount of fluid out to recenter the alarm sensor. But that was about a hundred years back and I would expect to find it different now??
In my thinking, this is a time to go for the pro truck guys as it is important to get it right. Going around with only half the brakes working is not good! I would try to take it by some of the smaller repair places that do brake work and just let them guide you on what it needs.
