ok ive got a 95 f-250 with a brake problem that has left me guessing. the problem is the pedal is soft and goes nearly to the floor, if u pump it you get more pedal but then the abs starts clicking. so naturally i thought it was the master cylinder,thats evidently not the problem. since the first master cylinder swap i have put 2 more master cylinders on , a new brake booster, changed the rear wheel cylinders, and i had 2 guys at my shop do nothing for but bleed the brake system for 8hrs straight. but still a soft pedal and no stoping power, (but once you push the pedal to the floor it will lock the brakes up..... so any ideas??
thanks in advance

 

There is a proportioning vavle along the frame that could be bad, or your abs valve may have air trapped in it.

 

I`m with Blue Rebel on air in the system. When bleeding the brakes make sure the rear brakes are fully adjusted up and make sure you bleed the RABS valve block on the leftside frame rail after you bleed the rear brakes.

 

Once had a similar situation, with a '75 Chevy van. Bled quarts through it, changed masters, to no avail.

Finally, I discovered that the banjo fitting at the L.F. caliper had a bad (or missing--can't remember) copper washer. The L.F. caliper was sucking air back in, around the banjo fitting at the brake line which connects to the caliper, and the air may even have been migrating back up to the master cyl. Whether it traveled or not, it made bleeding this van impossible, 'till I replaced the washer, and it bleed out fine then.

I can't remember right now if our trucks have these washers and banjo fittings or not--but the point is--doublecheck EVERY connection that has been disturbed in the recent work.

Also--are the BENCH BLEEDING the master completely, prior to installation? This avoids pushing mega air throught the system while the master cyl. fills....

Good luck.

 

ok thanks ill double check all the connections and bleed the rabs block... got my fingers crossed the old shop truck may have half a million miles on it but it starts every time and uses very little oil... hate to junk it cause it wont stop lol

 

Also looking into brakes.
I have been troubleshooting my brakes for some time now. Lazy K suggested I bleed the RABS. I've done that with no change.
Should I re-bleed all the brakes since I had forgotten the bleed the RABS first?
I've also noticed something else. When pushing down on the brake pedal, I can hear air passing through the vacuum lines, but it never stops. The longer the air pass, the more the brake pedal goes to the floor. Could this be a problem with the Vacuum Booster system?

 

My friend is a former Ford tech, who owns his own shop now. He told me I could simply eliminate the $140. RABS block, and replace it with a simple loop of brake line, which I did.

He explained that, on the '88 F-150 (and this may apply to other years--use this advice at your own risk) the "ABS" is not a true ABS system, for two reasons:

1) It only applies to the rear wheels.
2) It has no pump with which to "cycle" the pulses of pressure, like a true ABS system.

3) The RABS valve is not a real ABS system at all--that the RABS is basically just a chamber which allows pressure to flow into it, in essense "slowing down" the response of the rear brakes, to help prevent rear wheel lock up. When the RABS reservoir fills up, full pressure is (belatedly) supplied to the rears.

Which may be why the OP's rears do lock up, eventually.

I took that RABS out, and NEVER experienced any inappropriate rear wheel lockup--maybe that's because I have a heavy fiberglass cap on the back, with an 8' bed but again--no issues with removing it, about 3-4 years ago.

If your truck is newer, and has a real ABS system, then you should probably ignore this advice. Again, mine is an '88.

Hope this helps.

BigSix

 

One nice rainy night I shut my RABS off by pulling its fuse, then went for a drive and while flying down Telegraph Rd. I laid on the brakes good - basically I tried simulate an emergency stop on the freeway in bad weather. Result was interesting, but sorta expected - instant lockup of rear tires, rear end coming around, and truck sliding almost sideways taking up all 3 lanes. Yah, I put that fuse back in real quick If you wanna remove your RABS and run full pressure to the rear brakes that's your call, but I'd highly advise against it - the safe way to go about it is by installing either a factory proportioning valve from a similar pre-87 truck, or installing an aftermarket manual brake bias valve. I put in a manual valve in my old Chev, it was a super-light halfton that I converted to hydroboost and 1-ton master cylinder, I ran the rear brakes at 50% line pressure and they would still easily lock up if I got on them good on wet pavement, but overall braking performance was impressive to say the least.

 

Firstly, I want to say every time I try to type the letter "eff" to denote the model name of our pickup trucks, it gets translated into "Silverado."

I have a theory about this--allegedly, there are 8 million US computers infected, over the last few months, with this "Conflicker" or "Conficker" worm. AND NO ONE KNOWS WHAT IT'S FOR, OR CAN DO, OR WILL DO.

And it went active today, on APRIL FOOLS DAY.

So, I'm thinking this is some super-evil-genius worm, and it makes your own, personal nightmares come true. For instance, I don't drive Chevy pickups--I don't like them. So, every time I try to name the trucks I DO like ("eff series") it would type the GM counterpart--"Silverado."

And they said artificial intelligence was impossible--HA!

Well, that's my theory.

BigSix

 

M.L.S.C,

I hear you loud and clear, and do not doubt for one moment your "three-lanes-wide" experience. My short box 1972 Silverado—'er, “eff-one-HUNDRED series” (damn you, Conflicker worm-virus-thingy!) will swap ends VERY easily.

However, like I said, either because the '88 Silverado—“eff one-FIFTY-series” 2x4 (that I removed the RABS from) is an 8' box, and has a heavy, fiberglas cap, or for some other reason, it does not exhibit any squirrely braking habits, with the RABS removed.

When the RABS failed, however, I was having increased stopping distance/decreased brake performance. Literally, I could lock up my fronts while the backs were not yet locking--and then the backs would lock. I confirmed this on dry pavement, by hanging out the window, at night, to look. Yeah, it was a bit of a neck-snapper--LOL. But after bypassing the RABS--normal braking restored.

And in the 3-4 years since I've removed the RABS, I've made some full-on panic stops--trust me--I live in the tri-state area around NYC (CT/NY/NJ). I have plenty of opportunities to make "panic stops." This particular '88 Silverado—“eff-one-FIFTY” brakes very well, with no unusual, premature rear lockup.

And lastly, at least on the '88 “eff-one-FIFTY” (and presumably, related model years), remember that once the small, internal RABS reservoir does fill up and pressurize, during a panic stop, full pressure is sent to the rears anyway--my understanding is that that's the whole point of having the reservoir--to merely delay the onset of full pressure.

Why?

Because, since there IS NO ABS PUMP in these years of trucks, there's no way for the RABS to actually "pulse" the rear brakes on the edge of "lockup," as does a true, 4-wheel ABS system. RABS doesn't "pulse" the brakes at all—again, it just delays the full line pressure to the rears, slightly. (And presumably, that only occurs when the RABS senses a panic stop—either through actual decelerative forces, or perhaps high line pressure—I don’t know what it uses for a “signal.”)

So, my point is, RABS or no RABS, the rears are going to get hit with full line pressure eventually. Therefore, in a big stop, if they’re asked to stop too hard, given the forward weight transfer, they're going to lock up--RABS merely delays the lockup slightly, giving the operator time to modulate and back off of the brake pedal slightly.

Per my Ford tech-friend, that's the point of this RABS system--to avoid that initial, rear lockup-upon-weight-transfer. I think it's a flawed system at best. Frankly, I couldn't believe it when my Ford-factory-trained tech friend explained this to me. It seemed (and seemed) too simplistic to be of much use. He told me a number of his Ford tech friends, who drive these trucks, simply removed them.

Because think about it—with NO ABS pump, if the RABS released the pressure, to prevent premature lockup, the system has no ability to restore line pressure, unlike a true, 4-wheel ABS system. No way to do that…unless the pressure is only temporarily reduced, by “shunting it” off somewhere, like to the internal reservoir of the RABS valve body.

So, technically, I guess you could say RABS does "pulse" the brakes--ONCE--LOL! (Off, then back on full). Whereas, real ABS "pulses" them hundreds of times per minute, or whatever.

But you're right, M.L.S.C.--since we've both had trucks go sideways due to premature, rear lockup--I only avoided hitting a T-bird once because my “eff-one-HUNDRED” shorty turned sideways on me--removal of your RABS valve MAY MAKE YOUR TRUCK DANGEROUS. Howeer, in MY case, it worked well on the '88 “eff-one-FIFTY”. And, oddly, the 1972 “eff-one-hundred,” which HAD no RABS to begin with, had pretty severe rear wheel lockup from Day One.

Anyone who does remove a RABS should test their truck (as you did) and see how it behaves--for diagnostic purposes only--and then restore it to it's original condition. In my case, for whatever resaon, the braking performance was actually improved by the modification--YMMV.

 

Well, one thing to take into account, is that I drive a dually F350 with near bald rear tires, so my traction on wet surfaces is way less than what you have in your SRW truck with cap and all.

What happened to your RABS is that one of the seals inside it failed, so even with the valve shut there was still fluid leaking past it and into the accumulator chamber - there's actually a very easy fix for that, you remove the front cap of the RABS and pull the spring and replace it with a solid steel piece (read "bolt") of appropriate dimensions, then you put the cap back in - this way the piston can't move even a bit so all your pressure is going to the rear brakes as if you completely bypassed the RABS, but you don't have to actually remove the module and install new steel line with fittings in its place and rebleed the brakes afterwards.

And by the way the RABS does pulse, well sort of - while it does not have a pump to re-pressurize the brake line after it, it still cycles the valve like it would in a modern 4-wheel ABS system - the pressure rising again is due to you being on the brake pedal. In other words it will only cycle the valve and drain off fluid in the accumulator chamber till the pressure in the lines drops to a point where the rear wheels do not lock up. Think about it this way - front and rear brakes are not connected hydraulically, so the front braking pressure is directly proportional to the force you're on the pedal with. Assuming you're looking for maximum stopping power you will try to keep the pedal down just so hard that the front wheels are about to lock up but still roll, with an experienced driver this usually results in a pretty constant pedal force - at this point the RABS only needs to bleed off some pressure for the rears to unlock, with properly adjusted rear brakes you will never fill up the accumulator chamber entirely if you're attempting for a best stopping distance. Now Joe and Jill Regular will not stop like that - they're gonna stand up on the pedal all the way, till the front tires lock up, at about the same time the accumulator chamber also gets full but it makes no difference no more as once the front tires start sliding all bets are off and the only way of regaining control of the truck is letting off the brake pedal, which immediately drains the accumulator chamber and the cycle is ready to begin again as the driver panics and starts pumping the pedal.

 

by the way I can spell F350, F-Superduty, and PSD with no changes to GM models, however F15O, F25O, F - Series, and Power-Stroke do get converted

 

I think MLSC already covered this, but the RABS system does pulse the line pressure on the rear. It can't add pressure back due to not having a pump, but there is still pressure on the pedal from the driver. When RABS engages on my truck, I can feel the pedal pulse like any other ABS system, although it isn't quite as violent since the pedal isn't being forced back up. The panic stop "signal" comes from the speed sensor on the differential.

 

M.L.S.C.,

Thank you for the clarification, and I wish I'd known about your bolt trick before I cut in a loop--LOL!

I think I can see both you and Eric's points about some "pulsing" action being possible, due to further pressure being supplied by the still-descending brake pedal, but isn't that very limited, given that the front brake circuit is not going to allow the pedal to continue to drop, since the fluid in the front lines is not bypassing into a reservoir, unlike the fluid in the rear circuit, which goes into the RABS reservoir? And in the absence of a continued pedal stroke (because fluids are not very compressible), and given that there's no ABS PUMP, I'm not seeing how very much "repressurization" of the rear circuit, after the pressure bleedoff to the RABS reservoir, would be possible.

Weird how you could post certain "eff-serees" numbers, but not others. (When I tried to spell that correctly, the WORM wrote "efChevy". Weird!

And Eric--thank you for explaining how the RABS felt when working properly--mine never worked in my '88, so it wasn't missed when it started resulting in waay lengthy stopping distances, with the fronts locking and the backs doing very little.

I appreciate both of you explaining this system better than I could--I just know what worked for me, and I hope the OP can benefit from your more accurate explanations. And thanks for telling where the RABS gets its "signal" from.

Not to hijack this post but, since both of you guys are obviously very knowledgeable about these trucks, can either of you tell me if my '97 effa-TWO-FIFTY H.D. 4x4 has ABS? And is it a true four wheel ABS (with ABS pump) or only a rear wheel system? The OP mysteriously never supplied the Owner's Manual. Thanks!

BigSix

 

I'm pretty sure your '97 HD has the RABS, but you can check for yourself - you know what a RABS module looks like, you know where to find it on your '88, well it will look the same and be in the same place on the '97 as well, if you see a RABS module then that's what you have. Alternative trace the brake lines coming off the master cylinder, you know what a 4-wheel HCU with the pump looks like, if you find one in the engine bay then you got 4-wheel ABS, and not RABS.

About the brake pressure recovery with the RABS - you are correct, you can only build up so much of it due to the non-relieving front circuit "stopping" you pedal's movement, but that's just it, the RABS don't really need to repressurize - remember tis supposed to eliminate wheel lockup on you, it does that by dumping the pressure till the rear wheels start rolling again, then it stays there till you either stop, or let off the rake pedal. In most cases when the road conditions are fairly consistent (such as smooth pavement with no big oil spots) this is all you need. The pump was added to the HCU of later models to account for inconsistent road surface, such as chatter bumps, dirt roads, uneven city street pavement, and such - back in 2007 I once spent half a day driving a 2010 F350 mule over railroad tracks, I'd get up to 50mph then step on the brakes as the truck is going over the tracks (simulates road conditions with severely varying friction coefficients, for instance ice patches) then observe and record the ABS actions under different pedal force inputs. That 2010 had very aggressive ABS, which I'd imagine would make some drivers annoyed with the pedal pulsing so often, but it was very good at what it was doing, the wheel slip was negligible. In comparison the moment when your rear wheels get in the air (between the tracks) the RABS will dump nearly all the pressure, and when you land on the other side you pretty much only have front brakes - I experience that about every day when going to school, as just before the turn off the 50mph main street there was a speed bump (wtf were the city engineers thinking with that one), when I hit it as I was slowing down for the turn my rear axle would literally jump in the air causing the RABS to go into "oh sh**" mode causing a very sudden and big drop in braking force. A modern 4-wheel ABS will have zero issues handling that situation, but back in the late '80s and the '90s that was the level of the technology development, which is still way better than having no ABS.