Stock Brake Pedal behavior

[Tim McKay]

This is a 1993 IDI N/A 4wd with a vacuum brake booster.

Question: How does the RABII unit behave? Should the Stock pedal slowly continue to go down, almost to the floor, when the truck is running. When the truck is not running, the pedal sinks some, but not as far as when running (standing still). I wonder if this is a function of the RABSII unit? Filling of the dump resevoir that is described in the manual.
The pedal is high at initial pad/shoe contact. The truck stops excellent in a panic stop, stays straight. RABS tested on wet roads, seems to operate correctly, no rear lockup, the truck stays straight.

Here is what has been done to the brakes:
New Vaccum assembly, new hoses, distribution block confirmed no leaks. No leaks on other ends of hoses.
New Booster, Master Cyl, Check Valve, booster rod adjusted to the middle of the Ford spec.
Front Calipers and flex lines replaced 12 months ago, reconfirmed all working correctly.
Rear Wheel Cylindars, new shoes and hardware, brakes adjusted to lockup, then backed off, recenter shoes by stomping brakes, and final fine adjustment until slight drag.
Bled brakes in this sequence: RW, LW, RABS (truck running), RF, LF.

All this was done to resolve: 1) poor booster performance on cold mornings until truck warmed up. (New vaccum pump). 2) new Vaccum unit revealed hissing under the dash, leaking booster. ( might as well replace the MC while we got it off, otherwise it will go next). 3) Rear Wheel Cyl bleeder screws were rusted shut, couldnt get them broke open... might as well do a complete rear brake job. The classic “Peeling of the Onion” you always run into with this old stuff.
Of course I searched the forum and did not see this particular description of the issue, everything seemed to be caused by the rear brakes not adjusted.

Thanks in advance.

 

[Thewespaul]

Unplug the rabs and see if it still does it with the truck running. If it doesnt, replace or delete rabs.

 

[kpj]

My brake pedal goes slowly down also when running and it has been like that for ages. Other than that the brakes work as they should and they go through yearly mandatory inspection here and have passed, so i haven't been very concerned about it. I did once looked for a solution and found this post https://www.ford-trucks.com/forums/678376-brake-pedal-slowly-goes-down-tsb.html#post5387282, don't know is that causing the problem or not?

 

[Selahdoor]

My pedal now works the same as any other vehicle, in both my trucks.

In BOTH trucks, the pedal operated exactly as you describe, when I got them.

In the first truck, I spent 3500 dollars getting the brakes 'fixed', and they never got rid of that problem. I fixed it myself by bypassing the RABS. (Absolutely was NOT going to take the vehicle back to either of the two shops involved.)

In the second truck, I spent the same, but they honestly replaced almost the entire brake system. (Except the RABS, and the steel lines.)

In this one, when they were 'finished', the pedal was still acting like that. This time I took it back, and told them to fix it right. They did. It works now. I haven't crawled under it yet to see if they bypassed the RABS or just bled the RABS. Whatever they did, they fixed it. And I'm pretty sure it was in the RABS again.

They've decided they don't like my truck for some reason.

 

[idi_econoline]

Yeh, mine's been a bit odd since day one. I, too, have the hissing booster (replacement on garage shelf), and some pedal travel while hissing. Also on shelf are new m/c and rebuilt ABS valve, but will do booster first, with that hiss.

Mine also has worked fine, when I really needed them. But, I really should look into this... thanks for the nudge, gentlemen. I do get a rear ABS light now and then, but not often.

 

[chillman88]

In this one, when they were 'finished', the pedal was still acting like that. This time I took it back, and told them to fix it right. They did. It works now. I haven't crawled under it yet to see if they bypassed the RABS or just bled the RABS. Whatever they did, they fixed it. And I'm pretty sure it was in the RABS again.

They've decided they don't like my truck for some reason.

It's a shame, but I've noticed lots of shops don't like anything they have to "work" to fix. The main reason I do all my own work. It's hard to find a good mechanic. Every time I think I find one they do a "good enough" job instead of a GOOD job.

 

[Selahdoor]

Exactly. It was like pulling teeth to get them to even do the 'good enough' job on 80% of what I had it in there for. Forget the remaining 20%...

That was the bad news. The good news is that at least all of the major stuff that I didn't want to deal with myself, got done. Like the brakes. Now I am dinking with the stuff I like to do.


(Sorry about the minor thread hijack...)

 

[lakesurfer]

I had the sinking brake issue. Tried bleeding the system multiple times and even a special process to bleed the RABS. No help. Replaced the RABS (and bled the system). It got worse! Replaced the master, *being super methodical about bleeding & priming it on the bench*, then re-bled the whole system. That did it. Now, by the grace of God, I have a firm, non-sinking pedal.

P.S. Somewhere, in my research, someone advised that the E150 RABS is much cheaper, but was the same part but with an added plate. The advice was to just unbolt and discard the plate then install the RABS. This is what I did and it appears to work and be true. The part number I think I used was FOUZ-2B373-B. YMMV.

 

[Tim McKay]

I finally got a chance to run the test Thewespaul suggested unplugging the RABS unit. The results were inconsistant. Sometimes I would get a hard pedal, sometime sinking. This tells me something mechanical is going on. I ordered a new RABS, it will be a few days before it gets in. I will report back to complete the thread for others.

Lakesurfer, good point. This truck is over 8500 GVW, and thus carries a different part number from the other. I dont think it would have worked for this truck, but good info for others.

Thx for all the input.

 

[silexis]

I had problems with poor pedal feel and the pedal sinking. Regular bleeding didn't help. I ended up using a pressure bleeder to purge the system. That helped with the pedal sinking.
I also found my relatively new rear drums were out of round. Getting them turned really helped, it also eliminated the bed shimmying at high speeds. the brakes work great now.
The only issue I have now is that the parking brake will go on firmly, then pop down to towards the floor. Still seems to hold ok. When the drums were out of round the parking brake was crappy.

 

[Tim McKay]

Oh yea, the rear drums were turned also, I failed to mention that. They finished in the middle of spec., so they are good.
On another car I had a couple of warped drums, you could pump the pedal up higher, but it had a hard pedal at the bottom. A common result of warped drums.

Still waiting on that part....

 

[retiredinhomer]

how are the self adjusters working on rear brakes?

 

[Mountainman]

Hello All...(1990 E350 4x4 Ford van)hope what Im describing is you're problem....I had spongy brakes everything you described for 15 plus years...had brake shops do brake jobs....I did the brake jobs...it would be fine for awhile then spongy again...THEN I found out about this...leaking pressure around rear anti-brake solenoid which is underneath driver seat on frame rail...just trace brake lines easy to find...use (1 1/8 wrench I think). remove big bolt/cap carefully spring underneath cap/take out spring/put cap back on...and pump brake pedal/maybe bleed brakes and voila FIRM! been that way for 7 years now.

So spend money and buy new solenoid or just remove spring and no more rear anti lock brakes...which I do not need anyway and don't really work as advertised especially on old vehicles...just learn how to threshold brake properly and do not count on an antiquated system to do it for you..There Im off my soapbox!

Hope this is your problem! and it fixes it

 

[Mountainman]

PS...after this I also put on a hydro boost system which made brakes WAAAAY better...easy install...little bit of work making it fit and having hoses made...but well worth it...diesels do not make vacuum hence vacuum pump on engines but it still was not very good

 

[Tim McKay]

I finally got the ABS module in and replaced it.
I am getting the same results, so maybe A new module was a waste of money.

answering another question: the adjusters were in excellent shape ( truck has 155k miles on it ), we cleaned them up, put a little bit light grease on them, and they turn effortlessly. I heard them adjust once when backing up and stomping the brakes to confirm it. All other springs and hardware on the rear brakes were replaced.

So after completing what I will call a comprehensive brake job, the only thing not replaced was the brake lines, and they are in very good shape, I am sure this is a characteristic of these model brakes. Sad, but true. I concur with Selahdoor’s conclusion above. The only thing that probably did not need replacing in this comprehensive job was this ABS unit. Other component replacements showed signs of age failure, as outlined in my first post.

While I was waiting on the part to come in, I did some extensive internet searches. There was a lot of talk about Ford TSBs, but never found one. I did find this, which very well might be the TSB, as the wording sounds “corporate”. Other posts referring to a TSB were not convincing on their knowlege.

“1997 and earlier diesel trucks equiped with vacuum power brakes--Low/creeping pedal:
Many owners have complained of a low or creeping brake pedal on their trucks, often mistaken for a bad master cylinder or Rear ABS valve. If the condition occurs after the vehicle has come to a stop, and no other brake concerns are found, (worn or out-of-adjusted brakes, cocked pads, or hydraulic problems) the condition is normal and the result of the vacuum pump replenishing the vacuum inside the power booster. Ford has developed a Zero-Loss Travel brake booster to correct this condition if the customer finds it unacceptable. This part is only for use on diesel-equiped trucks--P/N F5TZ-2005-CA--and is not a service replacement; it must be ordered by the part number, not vehicle application.

This condition is similar to the low brake pedal concern on all 1995 F-series that required a larger-diameter master cylinder”

The part number listed is for a “Low loss” booster, that apparently is better sealed. I also read about a larger diameter Master Cylindar swap, as mentioned above. I already had the new MC installed, so I wasnt going to do that. Maybe I should have done some more research before starting the job. Maybe the results would have been better. The part numbers listed above are NLA, so I am not sure what to do about finding one of those. Todays auto part stores can cross crap.

I went through these pains as this is my sons truck, a young driver, and I wanted to maintain the ABS feature if possible, no matter how antiquated. He has yet to experience the joys of the rear end coming loose when you stomp the brakes on a wet road.
If he was more experienced, or it was mine, I would suggest considering deleting the unit, or modify it by removing the spring behind the reservoir piston. Overall, it was good for him to experience the process of troubleshooting throughout the system, step by step, and referring to the factory manuals during that process.

thanks for all the help.
After reading all those other sights, this is the best sight for help on these trucks. No jack legs with dumb comments.
Hopefully this comprehensive approach in one string helps someone.