Porportioning Valve

[WhiteGardens]

My RABS light is on. I replaced the sensor, and still no go. As far as I can tell, all the connections look good and have Di-electric grease.

I'm thinking of scraping the RABS system and installing a proportioning valve. I really don't want to drop 450 bucks for parts. Has anyone done this and what kind of results would I be seeing with a dually.

My only concern is the snow, but I usually go slow enough to let the motor do the braking in those conditions.

[Joe Mc]

I had hoped you would get some good replies on this post. But then I don't like abs and have often wondered how hard it would be to remove.

Duals and a short truck, you might want to keep some weight in the back.

[arthurgoboom]

If you're referring to that mechanical lever brake valve....mine's gone, and it's not bad in the snow that I have noticed, but I don't drive it every day either. I'd think your dump body would be plenty heavy to keep the rears planted.

[Joe Mc]

For the 1987 year Ford came out with a Rear ABS system to replace the lever used in the 86s.

[WhiteGardens]

Thanks for the replies.

Is the master cylinder a little bigger on the 87 also ??? Thought I read that somewhere.

I really want to do away with the RABS, Pete sent me some info on diagnosing the problem. I'll try to tackle that when it's not so hot out. If something is amiss besides a stupid electrical connection, then I'm taking out the RABS and not even fooling with paying to replace the components.

In order to plow in the winter, I put two tons in the dump bed for traction. Without having a posi, and being 2wd, it doesn't do to bad. The only time I have traction issues is plowing heavy wet snow, and even then I have a set of short chains to throw down to get traction if I get stuck.

So, needless to say I'll have plenty of weight going down the road, and when it's really bad, I usually don't go faster than 35, 40 tops. I also slow down way ahead of time and let the motor slow me down to hold traction with the tires.

So far, every mechanic I talk to thinks I'll be O.K. with the conversion as long as I get it adjusted correctly. I also would like to mount the knob in the cab so I can adjust it on the fly if I need to.

[WhiteGardens]

OK, so here is the new dilemma.

If, I use a proportioning valve, do I get an adjustable one, or a little more expensive one that has the automatic metering valves in them ??

[WhiteGardens]

Here it is, I'm hoping to have it in 5 days or so. I need to re-do the front brakes, and would like to get it installed asap.

Tilton adjustable brake proportioning valve IMCA NASCAR:eBay Motors (item 180375116596 end time Jul-05-09 19:46:19 PDT)

I'll re-post on here when I get it done.

Also, anyone done the F-450 hydro boost conversion?? Does the master match up to the booster ???

[cdnsarguy]

Does your 350 not have the adjustable load braking mechanism on the rear axle. I was under the impression there is an adjustable brake system for the rear axle. IE if you have a light load the you get light application of brakes....... if you load it up then the rear brakes compensate for the load.......

[Joe Mc]

cdnsarguy,

That is the system the 86 trucks had, in 87 Ford went to rear abs.

[1987IDI]

Originally Posted by Joe Mc

cdnsarguy,

That is the system the 86 trucks had, in 87 Ford went to rear abs.

I understand the lever type they are talking about from an old dodge i had, how does the rear abs work on the 87 and up? Does it have a wheel speed sensor? or is that what that little sensor on the top of the diff is for?

[WhiteGardens]

Yes, there is a speed sensor in the rear dif. The RABS proportioning valve is located on the frame under the drivers-side of the cab.

[cdnsarguy]

Joe Mc

The Height Sensing Brake Proportioning valve setup was still used on the Cab and Chassis trucks in 1993...... Whitegardens just has an older model.

[WhiteGardens]

Originally Posted by cdnsarguy

Joe Mc

The Height Sensing Brake Proportioning valve setup was still used on the Cab and Chassis trucks in 1993...... Whitegardens just has an older model.

Would there be a load sensor along with the RABS valve ????

[FMTRVT]

I’ve been staying out of this discussion as I usually just hang in the 99 and up forums, but I’ll throw in my $0.02 worth. And I'm going to get wordy while I'm doing something else.

I actually liked the mechanical prop valve located at the rear axle. When working and properly set up, it proportionally kept the rear brakes from skidding too early and provides reasonable full braking effectiveness. These were not only used on pickups, but Chrysler Minivans also used them. There are some issues though.

One is of course that it is mechanically working. The second is that it is setup correctly, which in order to service you need pressure readings from both the front and rear brakes, usually lacking in home projects and often lacked even at dealerships.

Another issue is that it’s not ideal. Disc brakes are very linear in braking. How much pressure is applied gets you corresponding braking. Duo-servo drum brakes are not. The output increases based on both the line pressure and the servo effects. So if you’re applying 2000 psi rather then 1000 psi, your going to get a whole lot more rear braking then expected.

Proportioning valves don’t account for this; even the older props located by the master cylinder or the adjustable ones. The older set prop may have been designated 300/.43. This means you have full pressure to the rear brakes until the 300 psi point, then the rear pressure is proportioned at a 43% of everything over 300 psi. In a hard stop at 1000 psi, you’re getting 601 psi with that prop.

What ends up in the engineering lab is that you make a compromise in the rear braking ability. You might design the system so the rear brakes will not skid worst case with no load in the bed, but then your not getting full braking under heavy load (weight preventing tire skid) that you could be getting. All of this depends on your expectations of how hard the driver will end up pressing on the pedal. If the truck is lightweight and he is making a close to panic stop, will he restrain how much he is pushing on the brake to the point of incipient skid and getting ideal stopping distance. Or is he going to be heavy footed and just skid.

The RABS was an attempt to correct for the both the set point prop issues as well as the mechanical issues of the height sensing prop valve. Adjust the pressure only when the deceleration rate is theoretically high enough that a skid would occur. Basically the software reads how fast the tone ring in the differential is slowing.

But I hate RABS. Drum brakes by their nature have a high degree of hysteresis– they don’t react fast to apply and release as disc brakes do. And neither does the RABS valve. Plus the RABS system does not incorporate a pump to regenerate the hydraulic pressure that it dumps. So each dump and re-engagement of the rear brake circuit requires volume from the master cylinder, so you end up with constantly dropping pedal while it’s doing its thing. The 4 wheel ABS just does such a better job. But it does end up providing a better stopping distance for those who are not that informed or trained about applying the brakes to generate incipient skid. That is if the prop valve is first selected properly from the factory.

To the question can you set up a proportioning valve in place of the existing unit? Yep, at your own liability risk. The common adjustable valves have been around for a long time, originally supplied by Kelsey-Hayes, which is now TRW. They are a simple valve, not like the combination valves of years past, where those valves included a pressure differential valve to limit fluid loss with a system failure, possibly a metering valve to slow application of the front brakes (allowing the rear brakes some time due to their hysteresis), and an electrical switch to light up a dash lamp to tell the driver of a problem.

The typical 2-port adj prop you’re going to find will have a set slope, like the .43 in the example above. What your able to adjust is the cut-in point, from 100 psi to whatever. They are not hard to install and we used to do this all the time when developing new friction material formulas for a new vehicle platform years ago before 4WABS. The new line is run from the rear brake port of the master cylinder into the cabin, then a return line back out to the existing rar brake line that used to be attached to the M/C. Connecting this up can be an issue, but NAPA stores carry WeatherHead fittings that allow you to do this. The guy behind the counter will not be able to match up parts, so it will be you looking through all the bins in the display to find what you need. The other thing you need to do is disconnect the existing prop valve and bypass it with hard line.

Here are some reference materials.

Proportioning Valves

StopTech : Balanced Brake Upgrades

www.tiltonracing.com/pdfs/98-1261_Prop_valves.pdf


Now keep in mind that your taking on the liability for how well your vehicle stops. Some S-O-B like me may be called in if you have an accident and run into someone, and the police, plaintiff, or insurance company sees what you have. And the acting accident reconstructionist will then evaluate if you did a good job of engineering your brake system, and if it had been properly adjusted the vehicle could have stopped 20 ft shorter and my client would not have been injured.


For your last question, there was no "load sensor". The mechanical lever connection adjusted the prop valve depending on the sag of the vehicle (bow of the leaf springs) between the rear axle and vehicle frame.

[WhiteGardens]

It's funny that you mentioned all the things in your post, only because of the lot of thinking I do, and trying to take most of those points in consideration.

So, if I'm reading you correctly, even with the adjustable proportioning valve, there is going to be the variables of driver input, and load weight.

I was looking at the Astro-van MC and Hydro boost and noticed that GM had a proportioning valve that (how I would describe) metered the pressure to the rear brakes in relation to the pressure of the front disks. If I've read correctly does this mean that it adjusts properly to compensate for driver input ?? This would probably be the better setup as it would leave less room for error with no manual adjustability.

I used to like my RABS until I had issues with the rear dif, and a couple of panic stops left me white as a gost. The last time I used the truck, I noticed that my RABS light wen out and I don't seem to have the break-fade that I had before.

I understand what you are saying about liability. Something that I didn't take into consideration. I would still think that I would have a little more peice of mind stopping with the porportioning valve knowing how it's going to react rather than a RABS system that has a mind of it's own.

My other thought is, that I want to mount the lever for the valve in the cab so I have adjustability on the fly, loaded or un-loaded. I also chose the lever style with 7 positions as you might loose the position of the knob-style more easily if you tweak it too many times.

I don't know, the way it is now, it's almost scary. I drive it like a dump-truck so my following distance is greater than average and I'm always at or below the speed limit, but once you need to stop in a hurry, I want that reliability.

Regardless, even if I install the manual valve, I'll need to do some thorough testing loaded and un-loaded to achieve the best results, and mark those positions on the valve. My neighbor is a big-rig Mech, so he might have something to measure line pressure after initial setup.

Potentially, I might also up-grade the rear drums to disk. I found a company out there that has designed a kit for the dually, utilizing a Lincoln disk and caliper. (usually only see the kit for the single wheel, so I was surprised to find the kit). If that happens I'll probably have to re-configure the prop-valve again.

Thanks for the post FMTRVT, if you've got any more thoughts or insight into this, by all means feel free to post.

Also, I don't think I have the load sensor/lever on my truck.