I have a 97 lariat. The brake pedal will start firm and slowly leak down. I can pump it once or twice, and get a firm pedal, that slowly leaks down again. If I'm sitting still at idle for awhile the pedal will bottom out and begin to roll within a minute. I've been searching the forum for a couple of days trying to solve the issue, finding no threads with the answers to the problem. I finally came to the conclusion, tha it would be the master cylinder, and replaced it today. I bled it on the bench, then installed it and bled the brakes, using the two man system. I'm still having the same problem.
When I turn the key on engine off, the ABS indicator comes on and goes off as it should. I can't find any leaks or fluid anywhere, and am not loosing any fluid. The truck has what I assume to be a RABS unit mounted on the fender well, with no leaks there either. The vacuum booster seems to be working right, or at least it has the right feel to it., and no vacuum leaks.
So what would be the next step to figure out how to fix the problem.

Thanks in advance for help with this issue

 

If you are convinced you don't have a leak somewhere then it must be another bad Master Cylinder... One of the seals must be leaking internally and that is why your brake pedal sinks.

How long ago did you replace it? I'd try going back and exchanging it.

 

I just replaced it yesterday. I'm sure it has been bled well, because we cycled a about a pint of fluid through each line, and also bled the master on the bench.
I've been researching the 4wabs system, and decided to unplug it today and see if the brakes still bled down. That made the condition much worse, and I still didn't get a MIL. So I've plugged it back up and will road test it in a little while.
Thanks for the reply. Any other ideas out there. I'm just not convinced the new master cylinder is bad also. I'm going to check fuses next.

This thread is the last one I've read.
https://www.f150online.com/forums/wh...-problems.html
It doesn't have same indications I have now, but I think I'll go through the fuses for lack of a better solution.
I'll try another Master cylinder later if I can't figure this out.


Thanks

 

bump

 

Does anyone have any experience with the 4WABS. All fuses are good.
I re bled the brakes one more time, using a vacuum pump. I cleaned all the connectors with contact cleaner. But I'm still not positive if I got a bad master cylinder when I bought the new one.
I would just like to be armed with more information before I argue with the parts store.

Thanks for any help and experience with this issue.

 

Have you checked all of your brake lines and hoses? Calipers? Rear wheel cylinders? (pull the rubber caps off the ends and check for fluid) A leak that bad should be pretty obvious. If all of those check out there isn't much left other than the master cylinder unless there is a way the ABS module/valve block can leak internally.

 

The system has never lost any fluid at all. It just will not maintain pressure. I've search several old threads, but there's just no clear fix for this problem.
I haven't found a thread where someone actually replace the 4WABS module to fix it.
I especially don't want to replace it, without having any codes for the problem.

 

Bad seals in the Master Cylinder.

 

Yep, it really sounds like you got a bad master cylinder. It's not that common but certainly not unheard of, 3 bad ones in a row is the worst I've heard about.

 

hmmmmm.... maybe someone should have suggested that sooner....

oh wait...

 

Hey man

Your check valve working as it should? Did you actually test the Booster?

The #1 problem with the half tons are the junk OEM rubber lines. This isn't your problem now, but those lines tend to screw other stuff up lol. Not kidding, a very weak link in these systems, -specially with twin piston caliper set-ups.

Anyway, if you Bench Bleed, you should be okay. Yea you have to use the two man bleeding procedure. A vac tool and auto bleeders haven't ever worked for me. I was told, "it should".... Oh well lol. I've used the power seat and a block of wood to bleed them if no ones around

Anyway, not sure? - Those guys are probably right ^^ up there, but I would check those components I mentioned and update the rubber lines if you have more than 50,000 on them.

BTW- If you need the Booster Check Procedure, I'll send it your way. - I almost positive the Haynes books have that as well.

 

OK, I'm a little slow. I'll try another Master cylinder, I just try to be careful about returning parts. You may not believe this, but I've made mistakes before by missing the obvious stuff.

 

The booster may be the problem also, It falls into the same set of problems.
But the pedal always returns to the correct position when I release it. I'm probably missing something here in this test. Shoot the link to me, and I'll give it a try. I have tested it the Haynes method, and it is possibly the problem. I'd like to double check it.

I found this thread this morning.
https://www.f150online.com/forums/wh...oes-floor.html

As I said Captain obvious gets me sometimes.

Thanks for the replies

 

Yea, there's quite a bit to test for the symptoms - proportioning valve in another. I sent you some info and have Service online. Looking at the procedures and what to test is basically all to do with power and flow. I gutted the 98 recently and replaced everything lol = From the wheels to the booster/ hard-lines, calipers etc.... Everything!! Even the calipers where fitted with STEEL pistons lol. I pull heavy quite a bit and NEED stopping power. Since I only have one set of calipers and drums in the rear, she needs to be perfect or better.. She is now - Stops on a dime with about 7000 in tow.

Like I've said many times in this forum, - The stock OEM rubber lines are junk and cause many problems. Braided lines are a must have for these trucks, -Night and Day difference/ better than new.

Well, just sent you a few procedures...PM me and I'll send more w/diagrams to an alternate location.

brew

 

Good info. Thanks a bunch jbrew. I'm hoping to get to it Sunday. Saturday if I'm lucky. But definitely what I needed.
I'll post up as soon as I check it out.