Hello All,

I have a 1999 F150 4x4 with four wheel ABS. The truck has had a sinking brake pedal since I bought it last year and I have been trying to fix it for the last five days.

Symptoms:
Pedal will pump up rock hard with the truck off but becomes mushy and can be pushed to the carpet as soon as the engine is started.

Work done so far:
New pads, rotors, master cylinder, all rubber hoses, and calipers.
Bench bled master cylinder.
Plugged outputs on the master cylinder and tested the pedal with the truck running, it is rock hard so I assume the booster and master are both fine.
Scanned for ABS codes, there are none.
Bled the brakes multiple times (I went through 5 quarts of brake fluid) and bled ABS module both with a scan tool and by cracking the fittings loose and letting fluid come out. I used a pressure bleeder as well as the conventional method.
Swapped in a junkyard ABS module and bled again, no improvement.
Did the youtube test of putting a screwdriver in the valve ports on the ABS and had my dad pump the pedal, the valves do not push out.
The system has no leaks whatsoever and had previously had all new copper brake lines installed.

I don't know what else to try at this point and the issue is driving me crazy. The only thing I can figure is that the previous owner got the line routing into the ABS module wrong when he changed the lines or that I got super lucky and have 2 bad ABS modules.

If anyone has had this issue or has any suggestions please let me know. Also, if someone would be kind enough to post a diagram or a pic of what lines go where on the ABS module that would be greatly appreciated.

Thanks,
Wes K.

 

Over time is it losing brake fluid at all?
After compressing them, do the calipers slide freely on their slides?

 

It loses no fluid at all. The calipers move freely and have new brackets and slides.

 

Going to ask this question? The master cylinder is supposed to be seated flush against the brake booster with a nut on the outside of the master cylinder and another nut holding the RABS bracket in place. Is yours like this?
Not like this: Does the mounting bolts on the brake booster had a nut between the booster and master cylinder and of course one on the outside of the master cylinder holding it on.
Okay, here is a way of making your brakes harder, but you have to be careful as it can set them to hard.
It is an adjustment that may have not been done correctly at the factory when the truck came out.

Here are the steps to try. They came from another forum site, and several people have tried it and been pleased.

-Remove the two nuts holding the master cylinder to the brake booster, they are 1/2in bolts.

-Once the nuts are removed, slide the master cylinder straight away from the brake booster. As I mentioned before there will be a lot of resistance due to the vacuum build-up. You can rock the master cylinder back and forth and you will hear the vacuum relieving, once it does you can slide the master straight back.

-Locate the rubber o ring and ensure it is still in place. It fits between the master cylinder and the brake booster.

-With the master cylinder pulled back (leave all lines and wires attached) just position it out of the way. You will clearly see a 7mm bolt that is threaded into the rod sticking out of the brake booster. The rod has a grooved shaft that you will need to hold with needle-nose pliers to keep it from turning while backing out the 7mm bolt.

-Back the 7mm bolt out of the rod (counter-clockwise) about 1/2 turn or how much you see fit. I tried half turns until I got the brake pedal to where I wanted it. It is possible to loosen the bolt too much to the point where your brakes will drag, that's why I went in such small increments. It took mine about 1.25 - 1.5 turns to get it right.

-Bolt the master cylinder back in place and you're done.

 

Thanks for your response! The master cylinder is seated flush against the booster but there is no other bracket. My truck has four wheel ABS and the unit is mounted to the frame down by the fender liner. Before trying this, I have already tried plugging the outputs from the master cylinder. With the outputs plugged and the truck running the pedal is hard and does not sink with instant brake pressure and almost no pedal travel. I assumed since this was the case the booster push rod adjustment wasn't the issue? If you think it is still worth a shot I'll go out to the shop and try it.

 

What have you got to lose at this point?
It is not that long of a job, but it has fixed a lot of other people's brakes. You just have to be careful.

 

You have a point. I'll try that tomorrow and see what happens.

 

Could be the ABS unit, the junkyard unit you installed has no guarantee that it didn't have the same problem as yours did.

 

Update: I tried adjusting the push rod bolt yesterday up to 2 turns out and it made no difference.

Alloro, I think that is the only option left at this point. I spent several hours yesterday and found that the ABS units are no longer available new as the company that made them is out of business. I spent quite a while looking through part catalog spec sheets and think I found an AC Delco ABS modulator that is nearly a direct copy of the old Kelsey Hayes unit. I have a new one coming in Wednesday from RockAuto and will update the thread if it ends up working.

 

I'm interested in hearing the outcome.

 

Okay, bad news and good news to report tonight.

The bad is that the AC Delco unit from RockAuto isn't even close to fitting.

The good news is that I figured out how to completely disassemble the original Kelsey Hayes modulator valve to clean it and change seals as necessary. I found several valves that were stuck from a combination of sludge, corrosion, and light rust. I practiced on the original one from my truck which was in the worst shape and will clean the junkyard replacement I bought and reinstall it in the truck tomorrow.

If this ends up working I will likely make another thread called " 1999 F150 ABS Repair " where I will provide step by step instructions with pictures for anyone else who has to do this. I think this will come in handy as these units are no longer available.

 

Update:

It turns out both ABS units I had were too badly damaged to make either one usable. I ended up finding a NOS one one ebay and it will be arriving by Friday. I will update the thread after I have it installed.