I am new to the forum and looking for help. I recently purchased a mutt of truck, a 97' F150 heritage w/5.4 Triton motor, 1/2 ton 2WD short bed.

Having a brake issue, the pedal keeps fading. I noticed it first while sitting in traffic on a down hill grade. I am holding the pedal and notice it starts to creep down slowly, I pump it up and it firms up then starts collapsing again. So general logic says hydraulics are loosing pressure right?....No leaks anywhere, so it must MC tight?


I have replaced the master cylinder, brake booster, and both rear wheel cylinders. Long story short they all needed it, truck has 265K+ on the body with 95K on the motor. The only thing I did not do by standard is bleed the MC on the bench, I installed and long bled starting with the right rear working my from longest distance to shortest (wheel speaking). So when bleeding I did of course get air coming out because of the new parts but I still have the same fade issue.

I am confident there is no air in the lines, I have bleed a quart of brake fluid through just to be sure. I have read a lot about proportioning valves and ABS stuff.

I need help, I will try and get some vehicle specific photos and attach have hit a road block with this. I am leaning towards the ABS valve thingy mounted on the drivers fender that has four lines attached (2 in and 2 out). Other than that it comes down to hard lines which have no leaks.

 

You need to bench bleed the master cylinder. The front-most part of the cylinder may still have air trapped in it because it's on an incline. It's the angle that the cylinder is mounted at that requires it to be bench bled. You can run as many gallons of fluid as you like through there and you aren't guaranteed to have removed all the air from it.

However, that will cause a low pedal, not a sinking one. A sinking pedal like you describe is usually either a fluid leak (sometimes the master will leak back into the booster) or a bad master cylinder. Sometimes not bleeding the ABS will cause similar symptoms though. I'm not sure but I think there's a bleeder screw on the module itself. I would try bleeding it. Do you have 4 wheel ABS or just rear ABS?

 

I bench bled the MC on the truck and made sure no bubbles left, then bled from right rear, left rear, front right, then left front. made sure no air coming from any of the bleeders and topped the reservoir off.

I attached tow photos, 1 showing the new MC and Booster and the other is of what I think is the ABS control valve thingy. Yes thingy, don't judge.

The pedal is firm if you pump it up or depress rapidly then begins to collapse to the floor slowly. No matter what I do the pedal just slowly creeps to the floor. I can pump it up and the pedal is firm then starts dropping.

I cannot tell it is 4 wheel ABS or just rear wheel, I do not see a bleeder anywhere. I have read a lot of threads and some have suggested to bypass but I prefer to keep the vehicle functioning as intended by the factory is possible.

Any suggestions are much appreciated. Thanks in advance.

 

Bench bleeding is when you bleed the master cylinder on your workbench. Specifically not on the truck for reasons I already explained.

 

Thank you for the follow up. I am confident the MC is clear of air. I started driving the truck with the brakes the way they are to get a better feel. I now am 98% sure it is the ABC control valve. While driving and depressing the pedal I can feel a slight pulsate at time or almost like the ABS is engaging then disengaging then the pedal goes back to dropping slowly until it hits the floor. And then I will have to bleed again and I will be taking the MC back of the "Bench Bleed" just to make sure.

I will post the results once I get the cash for the parts.

 

*ABS Control valve not ABC