The truck: 99 f-250 (feb 99), 5.4 auto, 4wd. rear ABS.

The story: Driving up north my ABS light came on. Stopped at a rest area with the kids. Turn the truck off. Light goes off. Light stays off the rest of the trip. On the way home it comes on again. Stopped again. Restarted truck, ABS light off. I Get stopped on the e-way in traffic and I stop fine. Once stopped the pedal drops in small, gradual increments till it basically bottoms out. ABS is light is off at this point. The pedal did this the rest of the trip. Checked for leaks at wheels and master cylinider. All is dry and reserve is full.

The FIX: This is where you come in...

Any clues?? Look forward to gaining your knowledge once again!!

Thanks, SteveO.

 

Internal Master Cylinder leak. That how it shows. Don't know if you can find rebuild kit, so most likely you will have to get whole new cylinder.

 

Thanks for the reply Kajtek.........I think.

Didn't know master cyliniders could have an internal leak! Hmmm, Learn something new every day.....

Off to the "replacing your master cylinider" thread!!

 

Strong indication of a master cyl. bypass.

 

Not so fast there. While an internal master cylinder leak will cause this type of issue, it will not set off an ABS warning. The other thing that will cause both issues is an internal leak within the RABS valve.

 

Driving home yesterday on the e-way, the crusie control shut off, the speedo was jumping all over and the ABS light came on.

VSS sensor in the differential......again? Had the same problem a two years ago but I don't recall the brake pedal issue.

Would the VSS mess with the brake pedal?? Or do I have two issues?

 

Had a similar problem on my 250. Under hard braking and while stopped - the brake pedal would creep... or drop before catching again. Replaced it with a reman'd cyl and the problem went away.

I didn't have a light however. Typically - the ABS light is a general warning that some sensor in the braking sys has a bad reading, usually low fluid. If the leak in the cyl caused the fluid to back up in the cyl or not return properly... it could have tripped the light. I've also had 'barely' low fluid - that expanded enough to turn the light back off while driving.

Not sure how helpful any of this is though. How many miles are on the truck? I don't remember the master cyl being all that expensive. It was a fairly easy swap, and would at least narrow down your hunt for the leak.

Hadn't heard of the valve being a possible leak though - would like to find out more about it, it's symptoms, possible causes, if there is an easy way to test, etc.

 

Speedometer jump changes things. Looking through the computer from far away, my guess would be two issues.

When you changed the sensor at the rear differential did you also replace the lubricant? The problem with the sensors is they get mucked up from the iron wear debris that collect on their magnetic surface. Occasionally they can be righted by just being cleaned off, but if the fluid has not been changed the cleaned or even new one will have an issue in short order. You have to correct the cause of the failure.

You still could have a RABS valve issue, but if it's the sensor that throwing the ABS light, then the probability of it being the RABS valve is lessened.

 

TR8,

If you do a Google search about RABS and pedal drop you will find many. many people with this problem along with a number of tests to check it. Here are two:

| Repair Guides | Rear Anti-lock Brake System (rabs) | System Testing | AutoZone.com

Kelsey-Hayes RWAL Antilock Brakes

There is another one which is more simplistic, but I can't find it on my computer right now.

I hate the RABS / RWAL system.

 

Fmvrvt, excellent information, exactly what I was looking for. Reps sent.

The more I read, the more I suspect the valve - but it's especially helpful that the links include a cheap and easy way to isolate the master cyl from the valve.

 

During the week I removed and cleaned off the VSS and re-installed it. Only had a chance to take it for a short test drive and all seemed well. No warning lights & no brake issues.
Drove 200+ miles this weekend without incident. No ABS light, no speedo bounce and no brake pedal drop. (even when I tried to make it drop)

I was pleasantly surprised. 1, I replaced the VSS about two years ago so I didn't think it went bad already (but sometime they do). 2, I changed the differential fluid last year when I had to replace the leaking differential cover so I thought the fluid was clean. There was just a little junk on it when I removed it. Thought for sure that wasn't going to be the fix!

Amazing how a little bit of junk on a sensor can affect so many things!!

Thanks for your thoughts and imput! I'm glad it was a simple fix................. this time!

 

Glad that seems to have taken care of the issue. Even when we change the fluid there are a lot of debris that cling to the walls of the differential and tubes, easily recontaminating the fluid to a lesser amount. Wiping everything you can and possibly a flush of the walls with some diesel or other light oil can help.

The same thing happens with motor oil changes, especially if you are doing oil analysis. We dump the old oil, run for 1k miles then do an analysis, finding that the oil is relatively high in contaminants after only 1k miles. It is due to the debris on the interior of the motor getting washed into the new oil. Not that the situation is catastrophic, just that it is improper to retake oil analysis that soon which will drive you crazy.