Serious brake pedal fade

You may have to bench bleed the Master Cylinder.

Your wheel bleeding technique may also no be so good.

(I do not know how you are doing it, but I have seen some real 'humdinger' ways!)

In my next link you will come across another link. This is how you can "Bench Bleed" with the Master Cylinder on the vehicle! (you can also pull up bench bleeding on YouTube, if you need another view of what is going on with the tubing, as it is maybe not so clear in my link). Doing it on the vehicle is so much easier!

https://www.2carpros.com/questions/1998-ford-f-150-cant-get-brakes-bleed

Do not run out of fluid while doing the wheels, Check the reservoir level every five to six bleeds.

Insure drum brakes are adjusted out so that the brake drum will barely go on, but not touch hard and drag the drum, do it by hand do not beat on them on.

Brake fluid will mess up paint, cool thing is, it is water soluble, so you can wash it away fast, clean the driveway too!

Return with good news!

The Medic

Oops. From the description it sounds like air in the ABS unit. Here is the book version if that is the case. I have made the special tools, just a flat piece of sheet metal with a notch on one end, fold it over and drill a hole through it for the screw. I have even seen them made from small hinges with a spring to open them.

You can skip item eight if you get a good pedal, it simply resets the valving in the unit.

MANUAL BLEEDING:
The Electro-Hydraulic Control Unit Brake Pressure Modulator Valve Module (EHCU/BPMV) should be bled after replacement or if air is trapped in the unit or system. If system bleeding is required, the module must be bled thoroughly before the wheel cylinders and calipers.

There are two bleeders on top of the unit that appear to be normal bleeders. These are modulator bleeders and must remain closed when the unit is not pressurized.

The internal bleeders are on either side of EHCU/BPMV module. The valves are used to open the internal passages within the EHCU/BPMV module. Both bleed valves must be rotated 1/4 to 1/2 turn counterclockwise before beginning the bleed process. The valve on the lefthand side is used for the rear brakes and the valve on the righthand side is used for the front brakes.

NOTE: The ignition switch must be in the Off position or false diagnostic trouble codes may be stored.

SPECIAL TOOL REQUIRED (or equivalent)

* J-39177, Combination valve pressure bleeding tool (three required).

PROCEDURE

1. Install combination valve depressor tool J-39177 to LH high pressure accumulator bleed stem of EHCU/BPMV module.
2. Install combination valve depressor tool J-39177 to RH high pressure accumulator bleed stem of EHCU/BPMV module.
3. Install combination valve depressor tool J-39177 to rear combination valve.
4. Ensure master cylinder fluid level, fill if required.

5. Bleed EHCU/BPMV module as follows:

1. Slowly depress brake pedal one time and hold.
2. Open left modulator bleeder, until fluid flows clearly or pedal is depressed, the close left bleeder.
3. Slowly release brake pedal.
4. Wait 15 seconds, then repeat steps until all air is bled from EHCU/BPMV module.
5. Close LH internal bleed valve. Torque bleed valve to 7 Nm (5 ft lbs).
6. Repeat preceding bleed steps for righthand bleed procedure.
7. Remove special tools.
6. Ensure master cylinder fluid level, fill if required.
7. Bleed wheel cylinder and calipers.
8. Turn ignition key to On position, then perform 3 function test with TECH 1 scanner.
9. Check brake pedal feel and braking performance; repeat procedure if required.

Given the age of the master cylinder, it was likely damaged if the brake pedal was run all the way to the floor during pedal-bleeding. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pushing the pedal to the floor runs the rubber lip seals over that crud and can rip them. That usually results in a slowly-sinking pedal, and that might not show up for two or three days. It can also make it hard to bleed the system because you will not be moving any fluid.

Triggering the ABS that way will not purge the air out of it because of the way the system works. When the ABS is triggered it only cycles using the fluid in the line the air just gets pushed back and forth.
When you use a scan tool to bleed the system it works a bit different and even then you may need to bleed it two to three times to get all the air out.
The method in the book holds open the valves that close when the system activates so you can push fluid through.

Some days handing it over is better than beating your head against the wall. We will still be here if you or your friends have any other automotive questions.

I have another comment that only applies to GM vehicles, and only those with split-diagonal systems. Maybe Steve or CJ can clarify if this applies to those with anti-lock brakes with a separate hydraulic controller or those with the controller bolted to the side of the master cylinder, mainly on cars.

For those with the split-diagonal hydraulic system, the front left and right rear brakes are on the same circuit. There is a valve in the master cylinder that trips when unequal pressures build up in the two systems. That valve blocks fluid flow to the two wheels that did not build pressure, presumably because of a leak. The same thing happens when pedal-bleeding. When you pump the pedal, then open one bleeder screw, that circuit will have no pressure, then, if the pedal is pushed over half way, the valve trips, and you'll never get fluid out of those two wheels.

Some people will tell you this can be avoided by bleeding in a specific sequence, but that is absolutely not true. If it were true, the valve would not trip if the first wheel in that sequence was the one with a leak. I only gravity-bleed, and I always start with the wheel I am standing closest to. Once all the wheels are bled and the bleeders are closed, "irritate" the brake pedal by hand a little, then open each bleeder once more for a couple of seconds to burp out the last remaining bubbles. This is even more effective after the disc brake pads were replaced because you need to work the brake pedal to run the pistons out of the calipers. Never push the pedal over half way to the floor. This will push any trapped air bubbles into the calipers where they can easily be bled out.

When you have those two wheels that will not flow any brake fluid, the only way I have ever found to reset the valve in the master cylinder is to loosen the cover on the reservoir, open one bleeder screw for a wheel that isn't flowing, then give the screw a quick, short burst of compressed air. Do not get so carried away that you push air all the way up to the master cylinder. You just need a tiny "pop" from a rubber-tipped air nozzle. If you left the cover tight on the reservoir, stand back, because fluid will be flowing from that bleeder very shortly.

I do not know if your truck has that valve in the master cylinder, due to having ABS, or due to not having a split-diagonal system. Your comment about being unable to get fluid to flow from one wheel is the clue that always pops up.

This would have a 3 way system. LF/RF/Rear. The tool I mentioned holds the valves in place so they cannot close and block fluid flow when the bleeder is opened and pressure drops on that circuit. The base brake system line failure valve on these is Front/Rear only. So he wouldn't get fluid out of either rear bleeder if it tripped.

Thanks, Steve. So to clarify, this is not a split-diagonal system. It is the normal front / rear system, but it still uses that valve in the master cylinder, including on those trucks with anti-lock brakes?

It is a front/rear split. Haven't seen a diagonal split in a while. At least not on a truck or most ABS vehicles.
They use a combination valve but it's not in the MC it's in the lines between the MC and the ABS module on some and inside the ABS module on others. The idea is that if there is any ABS related failure they still need the base brakes to operate. With a line/component failure the ABS gets shut down the second the combination valve trips.
Most still have the split piston MC parts as well. Just in case of failure.

Thank you. You're a wonderful human being!

Nope just a common grease monkey.

RICKYD1233
Please reply back once you get a confirmed fix.