RABS help! ASAP

[LCAM-01XA]

Well then, bypass the RABS (make a small steel line with fittings that you can bolt up the stock brake lines to without bending them), see if that helps any.

 

[MARQ2277]

Can I ask a dumb question? My brakes work awsome, I just want to know what a RABS is??

 

[Joe Mc]

Rear Anti-lock Brake System

 

[LCAM-01XA]

Aka the thing on the inside of your driver-side frame rail, under the floorpan, with a harness hooked up to it, and a brake line going in and another one going out of it.

 

[poolguy]

For what it's worth, I've had the same problem for a year now! I've replaced the master cyl 3 times w/new from Ford (even tried the F450 cyl, wouldn't fit), bypassed the RABS valve, bled countless cans of brake fluid through it replaced the pads, and one caliper, replaced & adjusted the shoes, and still have the exact same fading pedal I had when I started all this!
It gets a little scary when I'm pulling the back hoe or the track hoe (15K to 18K lbs each plus the 10 ton trailer). If it weren't for the trailer brakes, there would be no stopping it. I have to anticipate everything so far ahead that I'm usually cut in front of regularly. If the people knew what they were jumping in the way of, I bet they wuold stay in the other lane!
Anyway, if you guys know how to fix this I'm all ears! Throwing money at it hasn't helped.

Thanks,
Chuck

 

[supertom151]

Poolguy
I think you should look at your rear wheel cylinders, because even if they do not leak sometimes the rubber cups / seals inside the wheel cyl. get very soft and spongy and the brakes work like shmit, I think this happens more with trucks pulling heavy loads that over heat the brake fluid, it is also a good idea to go with Dot. 4 brake fluid it will take the higher temps with less brake fade.
The other thing to look out for is when you install a new wheel cyl. Make sure you have the right one, make sure that the piston bore of the new wheel cyl. is the same as the old one, if to small you have less braking power and if to big, the brakes can lock up in the rain without RABS.

 

[redneckaggie]

just rechecked the vacumm pump because for some reason 14 lbs just didnt seem right and it came out to 8 lbs. Is this to low? idk what was up with the other gauge.

 

[poolguy]

Poolguy
I think you should look at your rear wheel cylinders, because even if they do not leak sometimes the rubber cups / seals inside the wheel cyl. get very soft and spongy and the brakes work like shmit, I think this happens more with trucks pulling heavy loads that over heat the brake fluid, it is also a good idea to go with Dot. 4 brake fluid it will take the higher temps with less brake fade.
The other thing to look out for is when you install a new wheel cyl. Make sure you have the right one, make sure that the piston bore of the new wheel cyl. is the same as the old one, if to small you have less braking power and if to big, the brakes can lock up in the rain without RABS.

Thanks, I'll check them out. Since they weren't leaking, I didn't replace them. It's been on DOT 4 since I brought it home.
FWIW, this truck will not slide a tire under hard braking in the highway, no matter how hard you slam on the brakes. Even with no load.

Thanks,
Chuck

 

[Popeye2347]

I have got to 'jump' in here as I am having what sounds like the same problem, the previous owners comments were "They always stopped me!"....still gives me a bad feeling. Hope you find and post the solution soon, I will do likewise.

Also I might add, like others I have replaced the master cyl, found a broken adjuster cable on the drivers rear drum and a leak [axle] on the right rear. Hope to get it back tomorrow---fixed. We shall see.

 

[Timthetoolman]

I have always had too much rear brakes, I can stop by mashing the peddle so lightly the brake lights don't come on, the only time my ABS works is if my rear wheels are wet from rain.

Could it be the RABS?

 

[Popeye2347]

Hopefully someone with more knowledge than I will add to this, but as I understand it's operation, RABS only restricts fluid flow to the rear brakes, so unless you had a problem with the front brakes I do not see how it could cause the rear brakes to come on too strong, unless it fails to bypass when it should. Usually, from what I understand, it will not seal properly and act like a leaking master cylinder, yielding under constant pressure.

FWIW here is a link to a very good article on RABS. [Don't let the Bronco tag mislead you, it is pretty much the same for all Ford light trucks of this era.]
http://www.asashop.org/autoinc/dec99/techtips.htm

As mentioned there, you can always fabricate a temporary brake line to bypass the RABS for troubleshooting purposes. That alone could point you in the right direction.

 

[Compu Doc]

Bleeding brakes on the Ford truck with RABS.

1. Make sure master cylinder is full with clean fresh brake fluid.

2. Bleed the left rear brake first.

3. Bleed the right rear brake.

4. Bleed the RABS valve.

5. Bleed left front wheel.

6. Bleed right front wheel.

7. If there is still a low pedal or sinking pedal, disconnect the hydraulic lines on the master and plug the ports on the master cylinder.

8. Step on brake pedal with ports blocked on Master cylinder. If pedal is firm then master cylinder is good.

9. Reconnect lines to master cylinder. Step on brake, if pedal is low or fades RABS valve could be bad but before replacing remove back wheels and drums. Adjust brakes with the drums half way on so that you adjust brake shoes until they just hit the drum. You will see it by having the drum halfway off.

10. Install drums on fully, reinstall wheels and tighten lugs to the required torque.

11. Step on brake pedal and if brake pedal is firm and high then RABS is good. If the pedal still goes down and their are no other leaks than RABS is bad. Replace RABS and bleed rear brakes in the same order as mentioned above.

NOTE: When stepping on the brake pedal for the tests mentioned above make sure truck engine is running. Have truck in neutral if a clutch and in park if automatic. Also make sure wheels are blocked. The reason to have the truck running is so that your not pushing against the brake booster that does not have vacum. If you were to do the above tests with the truck off you could get false results creating bigger headaches and more expense by buying parts that are not needed.

Most of all do not jump to conclusions as to what is wrong with your brakes because you might be throwing your money away by replacing good parts with good parts.

If you take your truck to a mechanic, make sure he knows what he or she knows what they are doing and make sure that they know how to diagnose a problem correctly. If you do not then you will be the one paying in the long run as an incompetent mechanic will throw parts at it until it is fixed charging you for the parts and labor.

And yes I do consider myself an expert when it comes to the RABS because I have had to many experiences with people that do not know what the hell they are doing.

 

[oldmisterbill]

I tend to disagree with a few posts-Hope no body gets offended. On my 91 CC dually I had brake problems.Let me remind all I did was tow a 30 ft Gooseneck (sometimes heavy -sometimes real heavy 20,000 to 26,000 +) several hundred thousand miles. When I first got the truck I was impressed by the brakes. My brakes started fading when stopping,it got pretty bad & unsafe.Finally I lost them all together,luckily I was home.I bled them over & over no luck.I replaced the master cylinder got brakes next morning -no brakes-no fluid loss.Bled -bled (I always bled the RABS valve alsff to the parts store,tell him my problem he gives me another master cylinder-bleed-bleed,no luck.Back to store another master cylinder-same-same-(I had checked every thing else -rebuilt rear wheel cyls-manually adjusted rear brakes-calipers were not very old). Finally while bleeding I got brakes-wow! They would fade some if held but a test drive told me after stopping normal pressure on the pedal wpould hold it fine.
Off to Maine from Oklahoma with 3 tractors. In NY state -lost brakse to severe pedal fade. Bleed -bleed hours of bleeding-I got a spongy brake but a good stopping brake.several thousand miles in indiana -bad brakes.Went to a shop they bled -bled-bled -no brakes.I buy another master cylinder(by now I'm totally lost confused & mad as the devel)he got some brakes for me- off I got in the middle of the night (luckily I was empty) only 150 miles - no more brakes. Slipped home in the night 300 miles on my trailer brakes.
Another Master cylinder -no brakes-desperate -loads to haul and no brakes.To the books-study in detail how the RABS valve works! Quick bypass of the RABS (the bad part is I didn't have the fittings to adapt to the Ford fittings on the RABS) Cut -flare -bend -use a union.Bleed WOW! hard brakes! That was in 2003 still working fine now( I have changed the fuluid a few times).-almost jacknifed in a panic stop on a dirt Rd (rear brakes locked)-fortunatly there waas a road to the left I floored it whipped a left turn and slide some got it straight . Today the brakes are still fine-I sold my trailer,and now I occasionally drive it with no load (I almost never drove it without a trailer) I have too much brakes and want to put a new RABS valve back on.
You be the judges what was my problem??

 

[LCAM-01XA]

Bill, it seems like your RABS assembly went bad, there are two valves in there and the one that's supposed to hold against the brake line pressure until the electronics command it to open and bleed off some pressure (due to rear wheels locking up) sometimes starts to leak, letting fluid past it and into the accumulator chamber, resulting in fading pedal and weak braking. The way I test for that is open the RABS assembly's end and pull the spring out, then insert a solid sleeve in its place and close it back up - what that does is block the accumulator piston from moving, so even if the valve is leaking internally the chamber stays closed and the full line pressure is retained. If low brake pedal condition disappears, I then I just found the problem, if it's still present then I gotta look elsewhere.

 

[supertom151]

I got rid of the RABS valve and never had better brakes, I did that years ago and do not regret doing it!!!