So on my flat bed '91 F350 4x4 (the one the axle came out in April this year if you remember) I finally fixed it, new bearings and lock and all fixed and running great.


However it hardly has brakes.


New master cylinder, ALL lines from master throughout the truck to each wheel, new wheel cylinders, and calipers, and new pads all around, truck literally has a completely new braking system that I went through last November.

It had good brakes for about 2 weeks after doing the master and the lines, then pedal started fadding away again after 2 weeks of daily use, but I kepted driving it like that, didn't wanna mess with it in cold weather. Drove like that from Early Dec. to early April when the axle came out. Sat all summer, and just fixed it now and want it back on the road.

It brakes good, but near the floor.... and if I hold pressure on the pedal, it'll fade all the way to the firewall, it feels like it has air in it.


I went through a GALLON, yes a GALLON of brake fluid today bleeding everything. Both rear wheels, the ABS box in the frame rail, and front. Did it over and over all of it.

Never made a single difference. I did bleed the new master well when I installed it, and worked great for 2 weeks. No leaks anywhere, fluid never went down, still looks clean and new.



Rear shoes are adjusted as tight as I can go without to much drag on the drums, the little pin in the booster that pushes in the master cylinder is as tight as it goes without pushing on the master without touching the brake pedal.

I don't know what's wrong other than a bad new master...? When I don't understand is it worked perfect for 2 weeks or more when I first put it on.


Ideas?

 

I'm gonna say master cylinder, crappy new part.

 

Agree. On my fathers F150 we went though 2 aftermarket master cylinders until we bought a motorcraft one. Both had bad pedal fade

 

ABS module. There is an accumulator in there that when it fails you lose the pedal. For testing, you can put a plug in place of the rear brake line (front line off master) and see if you get a pedal.

 

I'll try that, cause I just replaced the master, and still no pedal.



I heard somewhere theres a way to bypass that damn ABS box by removing a nut on it, and theres a spring behind it you remove, put the nut back on and no more ABS.

Would that fix the problem?


If so, where abouts is that bolt or nut on it? Hard to see anything on this one, it's very rusted.

 

I'm not sure. It'd probably be better to just put a regular old proportioning valve on it.

 

How are you bleeding the master cylinders? Bench bleeding? Or mounted bleeding? Mounted never works right. There's a specific way to bench bleed them to get all the air out of them before you ever mount them up.

 

I know how to bleed a master, and yes it's on the bench. Master bleeds fine.

 

So update on this. After hours of googling and forum searching, I was beginning to think my RABS valve was bad.

I read lots of different ways of disabling it, disconnecting it etc.


After abit of thinking, I told my self, I have a good brake pedal with the engine off, doesn't sink at all, no pressure lost etc. As soon as you start the truck, it sinks, even before it starts building vacuum.


So I just tryed removing the RABS fuse from under the dash, and it helped alot! Pedal is still a little lower than it should, but doesn't fade much anymore.

It brakes great now, and not in the floor. So I think I'll leave it like that for abit, then once I get more free time in daylight hours, I'll remove the big hex bolt from the top and remove the spring inside, and if that doesn't bring it back up, I'll bypass that damn thing all together with a small brake line.

 

Mine eneded up being stripped auto adjusters on the rear drums. As soon as I replaced them, brakes worked great with no bleed down. The RABS was dumping pressure because of the adjusters causing the pedal to go to the floor.

 

bypassed my rabs completely, no more problems since!

 

same with all of mine. questionable brakes, you did not know if they were going to work or not. bypassed the rabs and never had any more problems.

 

I've been studying up on this so I'll know what to do if it happens to me on a long trip/towing or in a remote area. SO, basically I have to cut the flared ends off of the two metal brake lines going into the RABS valve with a tubing cutter and then splice them together with splice fittings from a parts store? Is that correct? Do I need an extra piece of steel brake line to do this? (I assume so). I just want to be prepared in advance..

I had to splice a brake line after an emergency stop on a highway in my very heavily overloaded F-600 dump truck with a 16 ft. bed. I'd already lost one tire tread just before that happened. A rear brake line exploded but at least I didn't hit the old geezer who pulled onto the highway in front of me. I expected he was going to do that so I was ready. Just vacuum assist brakes with a proportioning valve on that truck but they were so good that empty the brakes would nearly put you through the windshield if not careful. That repair held up for years until I sold the truck.

Anyhow, I LIKE ABS. This is a very important safety feature that I do NOT want to be with-out. So, I would fix it and advise anyone else to fix it. You might be more concerned with your own convenience than your own safety but what about the lives and safety of others out on the road?

I am very conscious of brake safety for very good reasons. I've been driving since (more than) 50 years ago and I know what can happen with malfunctioning brakes. Innocent people get killed by vehicles with malfunctioning brake systems, even whole families, mothers with kids, that kind of thing and the bigger the vehicle is that has malfunctioning brakes the more destruction it can do and is more likely to do because of a malfunctioning brake system.

Think of what would happen in an accident investigation like that when the investigators discovered that the RABS had been bypassed. Compared to no accident or lesser accident with a functioning RABS. They could and perhaps would consider that negligent homicide or manslaughter. It certainly wouldn't sound good in a criminal court or a civil court (law suit).

I consider these trucks to be large vehicles and to bypass the ABS system is knowingly creating a malfunctioning brake system in a large vehicle. As an example, without RABS your rear brakes WILL lock up before your front brakes and if you are in a turn on a slippery wet highway and need to brake moderately hard your rear brakes will lock up and you could likely skid out of control spinning into an oncoming traffic lane or into a ditch or off the side of a mountain depending on which way the turn is. That is just one example of why these trucks have Rear Anti-lock Brake Systems. It doesn't matter what kind of roads or conditions you normally drive in, at some point you will need that RABS.

I don't mean to sound like a jerk or be offensive but I think it's just plain good sense.

If you bypass the RABS and later sell the truck you had better make that very clear in the bill of sale or otherwise if that person gets in a serious accident guess who can get sued (and lose) for bypassing the RABS? If you make that clear in the bill of sale then the buyer is responsible to fix it and assumes the risk of knowingly driving it that way.

I wish these trucks had 4 wheel ABS but Rear ABS is better than no ABS especially in a pick-up truck. My dodge Dakota had 4 wheel ABS, I tested it on slippery roads and it worked great!

So, that being said, there are RABS Flash Codes (number of blinks of the yellow RABS warning light) and diagnostic steps to follow in the shop manuals eg. the '...Body Chassis Electrical' manual, in order to figure out what is wrong with the system. I was just reading the diagnostics for those Flash Codes, it might be a shorted wire, a dirty connector, a bad valve or a bad RABS computer, etc. There is a 14 pin connector where you can do diagnostic tests with a multi-meter...

BUT, I don't know how to activate these flash codes. Can anyone tell us how?

So, IDIDieselJohn, as rusty as the truck is, it could be a bad wire connector or shorted wire coming from the RABS computer. On my 87 that computer is located above the brake booster according to the diagram in the manual and I THINK it is at that connector where you can test the system.

It's probably the valve- I don't know. I can understand bypassing it temporarily from necessity but I would recommend planning on getting it fixed properly in the future if you intend to use the truck. I also read a comment indicating that if you bypass the RABS and one of the rear brake lines fails you will lose brakes to both sides instead of one side. Those rear brake lines look vulnerable to damage near the the rear wheels.

I appreciate this topic because now I'll inspect my RABS and I will be prepared to bypass the valve for a temporary fix if it fails at any time.

 

I have had to panic brake with it bypassed. fronts still locked up first. New rabs unit is pricey. and not very effective at its job. it was a poorly designed system to begin with. All i did to bypass mine was by a union with the appropriate size and thread on the input/output. bent the line around to match up with the other. simple fix. now i dont have spongy garbage brakes and can actually stop while feeling confident in doing so. I have no plans to sell my truck to someone else. If i did i would be sure to let them know.
edit: RABS or no RABS there is only one feed line that Tees off to the wheel cylinders. Fluid takes the path of least resistance. either way both rear brakes will be useless if one side or the other has a failure.

 

Thanks for that 'fix' information and the good sense in your post and a few others who have bypassed RABS. It's been so long since I've spliced a brake line I forgot what the parts are called. Also correcting info about the rear brake lines if one breaks. I had been under the impression that the Tee on the rear axle is part of the RABS, I saw it labelled as such somewhere, possibly in a brake bleeding diagram. It would have to be a valve to isolate a broken rear brake line. I just looked at mine and it has a hose fitting on it which is probably for flushing or bleeding the system in front of the rear lines. What I had read about isolating a broken rear line was in a forum about newer Ford trucks than the IDIs and also gasoline powered trucks but RABS appears to be the same or work the same on gas trucks. Maybe the parts are the same and if so that adds a lot for wrecking yard options.

[Please read all of this post]
I must apologize to IDIDieselJohn and others for that overly concerned post I originally wrote. OBVIOUSLY it's better to have working brakes at all 4 wheels than a defective RABS when they are not easy to diagnose and expensive to replace with new components. I woke up this morning regretting being so critical in that post so I'm here to edit it but not sure how I will handle that. Seriously, I had been up all night the night before and instead of sleeping in the daytime I forced myself to stay awake through the day into the night again so I could get my sleep schedule back in order. (insomnia caused by chronic pain issues). I should know better than trying to write when I'm in that state but it's a lesson I can't seem to learn.

I had also just spent a couple of hours reading about trailer towing accidents and their causes so my mind was in that 'safety first' mode as well as being almost in a dream state when I wrote that.

Today I wondered, "What would a professional brake shop do with a truck like this with a defective RABS and the owner could not afford to replace those parts?" Would they send that person on his/her way with the bad brake issue or would they bypass the RABS for essential safety?"

I know quite a bit about Law and I'm quite sure there is a way that a professional brake shop could legally bypass the RABS with some kind of legal form for the owner to sign to eliminate the shops liability in such a case which is certainly better than refusing to service an unsafe truck and sending the owner down the road. They could still offer a warranty on the brake work and exclude with a special note the RABS system. THAT makes the most legal sense but I don't know how many professional shops are prepared to do this. It's probably made unnecessarily complicated by lawyers and legalese 'mumbo-jumbo' but lawyers do that to cover every possible scenario for the clients and to keep the language out of the range of understanding of most common people so that only lawyers can deal with such issues (job security in the "legal profession welfare state" - which it is.)

I just called one of the best top rated service shops in the County where I live and discussed this specific RABS issue with the service manager. She was not immediately aware of what the Law would require in such a case and said that would have to be researched. I do not live in a County that has any large urban areas or bigger cities. Perhaps I should have called a national chain for more clear advice on that.

She did say that if there was an accident and the brakes were found to be a cause then their shop would be liable so in her view they would not bypass the RABS. She did suggest that re-manufactured parts at less cost is an option that she would recommend in such a case and she referred me to BBA Re-manufacturing for ABS parts, controllers, etc:
BBA reman. ABS, ecu's, throttle bodies, air mass meters, MFD and catalytic converters

That being said, knowing the parts numbers and the interchange years there are probably a lot of these parts that are still functional in wrecking yards for very low prices - especially the U-Pull the part type of wrecking yards. I am assuming that these RABS units are the same as in many of the gasoline vehicles of the same era and if so they should be easy to find. We just need to know how to test them to make sure they are good. Keep in mind that the IDIs will usually have much higher mileage than the gas trucks so if they are interchangeable the ones on the gas trucks are more likely to be still in good working condition. I don't have the time or convenience to search out those part numbers, specific vehicles and interchange years. Maybe someone else can.

I have read two comments in other topic threads/forums where those posters have stated that that the RABS works quite well. One discussed actual skidding tests that he did on an icy road surface and the results were very impressive!
As I recall, he said that if the rear end slid out it would correct itself and come back in line to follow the front end to keep the truck straight (that's impressive) and if the front brakes locked up in a skid to the side the the Rear ABS would also correct that and put the truck straight again (that is even more impressive!). The latter makes sense if the rear brake tires are maintaining better traction than lockup up front brakes. I think both comments were in gasoline powered trucks but that should not make much difference. The discussion was related to RABS so I hope that second tester's comments were in fact for RABS rather than all wheel ABS. Since he mentioned locking up the front brakes he probably did have RABS.

In view of those comments I am NOT going to accept the rationalization (or convenient excuse) that these 'RABS were poorly designed.' They may not be as effective as more modern all wheel ABS but from what I have read about pickup trucks of that era the Rear ABS was the most important to keep a pickup truck straight in a skid and that is why they were equipped with RABS.

That is my conclusion at this point so I'm inclined to leave my original reply as it is.

Finally: Why would a person need RABS in a warm dry climate on dry roads? If you've ever lived in an urban area like Los Angeles (warm dry climate) you would know why. The streets in high traffic areas are coated with exhaust residues and oil, transmission fluid etc. from vehicles with fluid leaks ESPECIALLY near major intersections, traffic lights and stop signs. After a very light rain which is not uncommon those wet surfaces on top of those residues become very slippery especially in the vicinity of intersections with traffic lights or stop signs. I remember standing at a big intersection in Beverly Hills right after a light rain watching the cars skid to a stop when the lights changed to red. That continued for a short time until the pavement dried out from the traffic. That was before ABS was invented. I'm sure there were many 'fender benders' mostly at intersections back in those days due to those conditions but the similar could happen anywhere on those roads. Just the exhaust residues alone are enough to make wet roads slippery for awhile.

I HOPE that the RABS components in gas trucks and even other gas vehicles of the same era are interchangeable with the IDIs because that will open up a huge range of options from wrecking yards. I think the valves would at least be the same if not the controllers.