93 F-250. 7.5L, 5-speed 4WD.

The truck sat for a number of years before I purchased it, I'm assuming the sitting and age are what are fueling my current issues.

So... the Brake Fault and ABS lights come on with some frequency. Not necessarily EVERY time I drive, but certainly every other. I noticed today that as the fault lights came on in the dash I nearly got a Kia or Hyundai suppository...

NO BRAKE LIGHTS!!!

Hazards and turn-signals work as advertised, so I threw a multifunction switch at it. Now I have brake lights, but they come on so deep in the pedal stroke as to be nearly worthless. I cannot come to a panic stop every time I need brake lights.

I'm not the diagnostician I wish I was, I plan to go ahead and throw a brake light switch at the truck tomorrow and have a Cardone remanufactured ABS hydraulic unit waiting on me at the local O'Reilly's, that will depend on the advice of the community here.

I need a slightly better understanding of what trips the two brake fault lights before I wind up replacing the entire brake system. I have a strong suspicion that I have a blown wheel cylinder as well, which may be the underlying issue with the brake light switch??? That's another problem for tomorrow Jeremy.

I know that the master cylinder level float will set off the brake fault light, I'm not certain if low brake fluid level will also trip the ABS light???

Where does this painfully rudimentary RWABS system pick up its inputs from??? VSS is the only obvious input to the system, I'm assuming there's something in the hydraulic unit that senses pressure??? Perhaps the suspected blown wheel cylinder isn't allowing the rear brake circuit to develop full pressure??? Which possibly trips the fault lights???

Anyhow, I bought this truck mostly because I felt confident I could keep it running far more easily than a more modern vehicle. Every now and again my confidence gets shaken... I have NEVER, EVER had the brake issues I've had with this vehicle with any other of the three or four dozen vehicles I've owned.

So, awesome Ford-Trucks.com community... please help me understand this system a little better. Obviously I'll need to get to my shop and pull the rear wheels off to see if I have brake fluid leaking out... if not there I'll have to look closer. The mechanics of the brakes do not feel right at the moment.

Sorry, it's been a long day and I'm a little fried. Hopefully I've given all the necessary data points, so on to my questions:

1) What, beyond the VSS, informs the ABS hydraulic unit???

2) Will pressure out of a set threshold trip the brake fault lights???

3) Aside from disabling the ABS hydraulic unit by removing the spring from under the cap, is there any other way to diagnose a fault in that unit??

4) Has anyone just gotten rid of the hydraulic unit entirely?? I was thinking of just replacing the unit with a length of brake line... of course Ford foresaw that potential and made the lines two different sizes, so it's not as easy as that...

5) Is there any reason to be examine the ABS "computer" behind the glovebox??? I believe I have another SRW "computer" from another OBS project, but don't need to go throwing any more darts blindly.
6) Is there anything I'm missing??? Probably should have been the first question...

Anyhow, thanks in advance, this community has yet to let me down!!




Be Safe




Jeremy

 

All I can say with any certainty is I doubt that you have a blown wheel cylinder or you wouldn't be driving long and would have a big brake fluid mess at a rear wheel and would soon be out of brake fluid. If you don' see any brake fluid leaks of any size anywhere I would try adjusting the rear brake shoes tighter if the adjuster wheels are not stuck. Otherwise I would tear into the rear brakes and inspect. That's all I got.

 

As stated above, remove your rear drums. Clean the parts and the self adjusters. Lightly lubricate the self adjusters.
Take a video of your brake components before you disassemble them. You need to know what specific order the parts are installed in.
If you see questionable parts, rebuild kits with new springs are pretty cheap. Ask questions here if in doubt.
At the front brakes, the calipers need to be free to slide. If you have the slide pin type, they need to be lightly lubricated too. I'm not looking at what year your truck is right now, but there's a lubrication process for bolt on calipers also.
Get a helper and bleed the brakes thoroughly, starting in the back on the passengers side. Bleeding them is a good time to flush out the old fluid. Let it run out until it's clean during the bleeding process, then move to the next wheel.
The ABS valve has its own bleeder screw. Bleed there last.
Here's a video on how the brake light switch works.
I took my brake light switch apart and bent the contact strip, so it makes contact sooner. The green line in the picture represents the contact strip, inside the switch.

I don't recommend taking this apart until it's the last resort.
There's a thread somewhere on properly adjusting your brake pushrod, where it contacts your master cylinder piston. It's important to get it right. Too loose and you won't be applying enough pressure to the brake system. Too tight and your brakes will drag.
Do all this, then troubleshoot the ABS system.

 

I would check this last if you still have problems after inspecting the rear brake components, the front calipers, bleeding the brakes and looking at the brake light switch.
Do it one step at a time.
https://www.fullsizebronco.com/threa...ctuall.527217/

 

Alright, I started my day picking up the new hydraulic unit waiting at the local parts store. From there I pulled the truck into the shop and pulled both rear wheels off. What I had interpreted as a potentially leaking wheel cylinder last night, on my back, in a dark gas station parking lot turns out to be unrelated:



Looking closer I found the reality, yeah, it's SOMETHING oily, but dripping down from the bed:



I hated having to cut the bed even more after I made this cut too deep. In any event it's pretty clear that's the source of the wetness on that side. Everything under the brake drum looked fine on that side.
On the passenger side there was some suspicious wetness at the wheel cylinder:


Short, controlled bursts of brake clean weren't touching it... two cans later and a bunch of brake pedal pumping couldn't reproduce the wetness:


I'm not sure where the wetness came from, but NOT, apparently, from a leaking wheel cylinder. I peeled the dust seals off both sides of the wheel cylinder, underneath was dry on both sides.

Even so, I left that wheel off and moved on to the hydraulic unit. I've taken the original hydraulic unit apart at least twice. My initial brake woes in the truck stemmed from degrading rubber lines. There's a screen at the inlet of the hydraulic unit both times I had the ABS light trip before it was accompanied with a noticeable decrease in brake performance. I told myself I wasn't gonna attempt to clean the old unit a third time...
The cheap-O vacuum bleeder I have worked overtime today. Starting at the farthest point, passenger-rear, I ran a gallon of fresh brake fluid through the entire system, front and rear. I'm not NASCAR-level, but I've been through the the brakes on this truck... and thankfully you can easily access all of the bleed-screws with the truck on the ground. With no wetness reappearing on the passenger wheel cylinder I went ahead and buttoned everything up.

With the engine running the brakes feel GOOD! Better pedal feel than I've had lately, no lights in the dash, but I'm still not exactly clear as to what the sequence is that causes the fault lights. Still, I sat there and pumped the brakes enough times to feel good about my work to that point.

Enough times to be absolutely certain I STILL did not have m*+herf**king brake lights!

SIDEBAR:

I happened to have a multimeter in the truck last night when I noticed I no longer had brake lights. I was on the way to take the kiddo to cub scouts, so I'll had already verified that most of the brake lighting circuit was working as advertised, I had power at the brake light switch... and the switch appeared to be working. Circuit open at rest and continuity with application of the pedal... I knew that I had lost a couple of the multiple functions of the multifunction switch, so that seemed reasonable. Swapping the switch in the O'Reilly's parking lot last night netted me brake lights... if I hit the brake pedal hard enough to lock up all four tires. Intermittent at best, so back to hitting the emergency flashers with each application of brake pedal on my way to the shop this morning.

BACK TO THE PRESENT:

After bleeding the brakes and buttoning everything back up I had lost even the intermittent brake light operation I had last night. Under the dash again... check for power... ring the switch for continuity again... still have power and the switch still rings. WHAT GIVES?!?

Long story short, another $11 to the parts counter person, another trip under the dash to pull the switch and install the new one...

And I have brakes now!!!

The original switch still seems to function EXACTLY like the new one and as advertised... both installed and on the bench... yet somehow it is still objective to say it was a broken brake light switch the whole time.

It's a good thing I really like this truck. I wouldn't put up with this crap from a General Motors product.

Still, I would like a better understanding of how exactly the RABS functions, where it draws its inputs and what trips the lights.



Be Safe


Jeremy

 

Sounds like you got everything functioning again.
Is it possible you have an axle seal issue causing the dampness inside the drum?
Haynes manual has the RABS information. Here are some screenshots. Don't know if they'll be readable. I've never pulled ABS codes.

 

ya gotta love chinesium parts.
just because it is new does not mean it is good. i saw 6 brand new starter relays bad out of the box.
counter monkey told me i had to be putting them on wrong, or the wiring in the truck was bad.
so i broke out the volt meter and showed him there was voltage at the trigger wire, and jumping across the relay engaged the starter.
i finally gave up after the sixth one being bad and jumped the relay to start it and drove it home and parked the truck until i could get a stancor relay delivered.