Northwoods Snowman

This is directly from the 2000 XJ Chrysler FSM page 5-3 (emphasis added):

PROPORTIONING VALVE
The proportioning valve is used to balance frontrear
brake action at high decelerations. The valve
allows normal fluid flow during moderate braking.
The valve only controls fluid flow during high decelerations
brake stops.

In other words, it does nothing unless you slam on the brakes. You're correct about the rear having an easier tendency to lock because of the weight transfer to the front and having more weight in the front in the first place. You're forgetting one VERY important thing though, the brakes them selves.

Our Jeep have disc front and drum rear. Given the same line pressure, the front are capable of stopping a lot more weight than the rear brakes. Do you know much about hydraulic principles (I'm being serious, not sarcastic)? If you do this you already know this, but the fluid has the same pressure throughout its entire volume. The piston in a front caliper has more surface area than the two small cylinders of a rear wheel cylinder. Since pressure = force / area (psi), then force = pressure x area. Since the pistons are larger they can deliver more braking force to the front discs than the rear wheel cylinders can to the drums. This is how you get a multiplying effect with a hydraulic brake system and you can have a lot of braking force with not much force on the pedal.

Lets do an arbitrary example. Say the master cylinder has a surface area of 0.5 square inches. The front caliper piston has a surface area of 3 square inches, and the rear wheel cylinder has two pistons that are each 1 square inch. The brake pedal is pressed with a force of 50 lbs. That means the pedal is applying 100psi to the system (50 lbs / 0.5 square in.). Now at the caliper because the surface area is 3 square inches, the force applied by the piston is actually going to be 300 lbs. (100 psi x 3 square inches). That just created a force multiplication of 6x. Now because the total surface area of the rear wheel cylinders is 2 square inches (both sides of the wheel cylinder) the force applied by the rear is going to be 200 lbs (100 psi x 2 square inches) or a multiplication of 4x. This is why higher performance vehicles often have 2, 3, 4 or sometime more "piston calipers" (as in "2 piston caliper" or "4 piston caliper"). The more pistons you have in a caliper the greater force you can achieve.

You're on the right track with your thinking, but you just don't have the whole story. Have you ever seen a 4 wheel disc brake system that had the same brake components on all four wheels? No, the rear is always smaller because of the fact that the front does most of the braking. The rear rotor is smaller and the caliper is smaller too. I had four wheel disc on my Subaru Legacy GT and the front had dual piston calipers and the rear was only single, in addition to the front rotors being larger than the rear.

The proportioning valve is not used to compensate for the weight shifting seen under normal braking, the design of the brakes does that. Like the service manual says, the valve is intended for high deceleration circumstances. Now just so we don't add more confusion over "high deceleration" I want to point out that this is going to be a slamming on the brakes situation. I don't know anyone (besides someone who can't physically react quickly) who would slowly apply the brakes until they were pushing the pedal as hard as they could in the event they had to stop VERY fast. If someone needs to stop that fast, they are going to be slamming on the brakes quickly, and that's where the proportioning valve comes into play; during that large rush of fluid from the master cylinder to the brakes.

Am I making sense? If you want my credentials for understanding how these kinds of systems work, I graduated with a 3.92 GPA from Michigan Tech University with a bachelors degree in Mechanical Engineering Technology. Not everything on howstuffworks.com is written by engineers that build the systems or people that REALLY understand what's going on. That paragraph you linked to only holds true when you have the same brake components on all four wheels, which is never the case. As I stated earlier, the front brakes are always larger/stronger than the rear.

Hope this was educational.

jeans

Sorry for the long reply time, its been a busy week at work....fixing hydraulics. lol

I may know a thing or two about hydraulic systems. Just maybe...I'm not going to name drop...but look very closely at my avatar and whats in the background...so, that being pointed out, hydraulics is how i earn my living. If i didn't know them, I would be fired damn fast. And then, it would be a waste to be fired because that means i would have wasted all that money in a college degree, and numerous dealership training courses.

Your math skills are quite excellent. And you have the fundamentals down pat. Your example is very correct from the theoretical standpoint. Yes, there is more surface area in the Front caliper pistons than the rear wheel cylinders, so yes, there would be more stopping power in the front over the rears. Mind you, your example doesn't take any other effects that comes into play during braking such as brake fade(heat), rear drum adjustment, weight transfer, and most importantly, the surface area of the rear brake shoes. Also, as far as my knowledge serves me, there isn't automotive application where there is the same braking at all four wheels (equipment on the other hand, there is.)

Something really bothers me about that statement from the FSM. That valve should balance the flow to the front and rears at all time(normal, i guess it would be called). There isn't really a "big rush" of brake fluid when you slam on the brakes. Its an increase of pressure. That pressure needs to be directed to the proper locations at all time, not just under "extreme braking conditions". If there wasn't any pressure being reduced to the rear brakes, there is still a higher tendency for the rear end to lock up. The lock up would be more noticeable under extreme braking because of the weight transfer. It would become really apparent when you start eating up rear shoes just as fast as your front brakes (as general rule of thumb, you will replace 2 sets of front brake pads for every 1 set of rear shoe/pads).

In the end, I think were arguing different angles/perspectives on the same point. I still think its not the safest route to fix the problem. I also think that the o-ring is needed to allow for proper metering of flow and pressure at all times.

I know that howstuffworks is most likely not to be written by engineers. Its a good reference point for someone who has little experience related to the topic. Its still pretty accurate to how it works. I know it doesn't give an example of the surface area on which its acting, but thats how it needs to work regardless of what braking system is on the front and rear. The numbers will vary from application to application, buts got the principal down pretty good.

That's my professional opinion, and ill stand by it.

00Pajeepcherokee

Just my .02 but when my brakes mess up an the rears get %100 along with my front ones it really isn't safe. Even on dry roads it can make u fishtail an flip a 180° real fast. (Ask me how I know.) Personally I'm glad my back brakes don't lock up as fast as the fronts because it is not safe. This is a good discussion an I appreciate the mature argument and I hope it continues. I'm being educated by the two of you!

Northwoods Snowman

Dang it! I just spend like 45 min typing a reply and then it got blown away when I tried to preview the post! ARRGGGHHHH

Anyway, I'll try again, this time I'll use Firefox instead of IE!

Jeans, I didn't intend to make any implications about your hydraulics knowledge, as I said "If you do you already know this." I'm also writing for the benefit of others reading this even though you and I are the two primary discussers (is that even a word?) Same thing going forward too.

I originally thought the proportioning valve would regulate the pressure too, until I took it apart. The front and rear fluids are completely separate and there is no "connection" or "bridge" between them. The fluid flows into the rear of the valve body (which is really just a big cavity), around the end of the brake switch piston, and out the bottom. Pretty much a straight shot through the valve body. The rear brake fluid flows in, past the valve, and then out. The only thing that touches both fluids is the piston for the brake switch, which doesn't move unless one side completely looses pressure. There are three O-rings on the switch piston and a V groove in the middle of it that activates the post on the brake switch (this is the brake warning light switch BTW for anyone that may be confused). One O-ring seals the V (which isn't sealed from atmosphere) from the front fluid. Another seals the V from the rear fluid. The third seals the incoming rear fluid from the outgoing fluid. The bore for the outgoing fluid goes all the way though the bore for the prop valve, and into the bore for the switch piston, where it just dead ends. So there is no link between the front and rear brakes for the proportioning valve to tell or "sense" what the front brakes are doing to adjust the rear.

That was the first thing I noticed. I spent probably an hour over all playing with the valve all apart and trying to figure out how it worked. I couldn't figure out how it regulated the pressure to the rear under emergency braking. They way it was designed, if the pressure goes way up from the master cylinder, the pressure will go up down steam too. The little ridges and bumps on the seal would still allow fluid to flow past it. That was until I figured out that it regulates the flow and not the pressure directly.

I don't know why you say "There isn't really a "big rush" of brake fluid when you slam on the brakes." There is, compared to normal braking. Being a hydraulics guy you know that the faster you try to push a fluid, the more friction there is and the more resistance to flow, thus more force is needed to move the fluid. Also, the longer the line/pipe/hose you have, the more of a pressure drop you see as fluid flows. There may not be a ton of fluid flowing in a brake system, but there is flow back and forth. If there wasn't the brakes would not engage and disengage. it doesn't take a lot of fluid because the pads and shoes are very close to the rotors and drums. The key though, is how fast that little bit of fluid moves.

Under normal braking, the fluid flows slow enough that it can pass though the little orifices created by the prop valve seal and everything functions normally. However, when you slam on the brakes, that little bit of fluid is going to try and get past the seal in a short amount of time. This sudden increase is flow is going to create a lot of back pressure in the line until the brakes engage and the pressure equalizes through the system. When this sudden flow of fluid and the resulting pressure wave hit the seal in the proportioning valve it causes the leading lip of the seal to flare out and reduce the flow of fluid. It doesn't cut it off entirely, but reduces it enough that the down stream line will see less of a pressure rise to hopefully keep the rear from locking.

When you sit down and think about it, it actually makes sense why they would design a system like that. I actually talked myself though it writing my original reply. You don't want to reduce the rear braking force all the time. When you are towing a heavy trailer, or have the vehicle loaded up with the family and camping gear, you want all the brakes you can get. There is going to be weight transfer under normal braking, no matter how much extra weight you might have, but worst case scenario is an empty jeep as it's the lightest. The engineers that designed the brakes designed the front and rear so that they would each respectively do their appropriate amount of required braking. They're not going to design the vehicle so front way over powers the rear or vise-versa. Of course the front will be absolutely stronger, but they don't want the front relatively stronger. What I mean by that is if the ideal braking is 80/20 front to rear, they don't want to design the brakes to be capable 90/10 or 70/30. The best system (and safest/cheapest/least liability) will be the 80/20. That's what they will design to. Just for comparison, the static front/rear weight distribution unloaded might be something like 55/45 or 60/40.

Now, that is under normal braking. When in an emergency stop, that's going to change. When stopping really fast, the front end is going to get a lot heavier, and the rear lighter. Say the braking needs then become 90/10. You suddenly have too much power (20%) in the rear relative to the front and can lock it up more easily. You're not going to get the back end light enough under normal braking to really need to worry about locking it up. It's only under really fast braking where you are slowing down REALLY fast that this is a problem.

This is where the design of the prop valve makes sense to me. You want all the braking force going to the rear, since the system is designed to have it (if someone doesn't design a balanced system, they suck as an engineer and company), when you're driving normally. So you're driving normally on a Saturday afternoon and everything is fine, until some jack wad pulls out RIGHT in front of you. You need to stop really fast, but what do you do? Do you slowly apply the brake as you would for a stop sign, but keep pushing until the pedal stops? Or do you suddenly slam your foot on the pedal while voicing some choice words the kids shouldn't really hear? I'd be doing the later (though I almost never cuss believe it or not). Now when slamming on the brakes, you'll get that extra weight transfer to the front and the need for less rear brakes and more front brakes. Also when slamming on the brakes as I mentioned earlier you'll be trying to push the brake fluid past the prop valve very quickly. This is where the flow regulation comes in. When that surge of fluid hits the prop valve seal and it expands, limiting the flow to the rear brakes (and this how quickly pressure builds), the rears are diminished in their capability. This will in effect give you the 90/10 ratio that you need in that situation, although technically your total braking capability (front and rear combined) is sightly reduced.

The seal doesn't completely block fluid flow as the fluid can still get past the ID of the seal. Also, since the seal expands to reduce flow, it's going to maintain a back pressure for a given period of time until enough fluid is past the seal to equalize the pressure. What this will do is keep the rear brakes weaker until either the pedal is released, or enough time has passed for the pressure to equalize in the rear brake line. When pressure does equalize, the rear brakes will be back to full force, but I would assume it's designed so enough time would have passed that the vehicle would be stopped, or nearly stopped anyway.

I know my calculation example was basic. I intended it that way. There are other factors like pad surface area, the kinematics of the levers and force vectors inherent with a drum brake system, adjustment, friction coefficient etc. There are just way too many factors to discuss here. People would and not have a clue what was being talked about anyway. I think the way to state that discs are superior brakes to drums so that most people will understand, is that there is a reason why performance cars use four wheel disc brakes instead of four wheel drum brakes, and why they can stop WAY faster than your average grocery-getter.

fantic238

Couple of questions: have you bled the slave cylinders and adjusted the shoes so that the drums fit almost tight? Did you roll a couple of 100 miles and re-adjust the shoes?
Have you checked the braking power with the hand brake partially pulled?

Northwoods Snowman

Have you read the whole thread? Your questions were already answered.

mdtobe

ABS has saved my life. Twice.

And concerning the confusion regarding the proportioning valve, as Northwoods Snowman wrote (and as the FSM states), its job is to prevent rear wheel lockup when the brakes are applied hard, by limiting the flow (and therefore for a few moments, the pressure) going to the rear brakes when one jams on the brake pedal hard. I have been having the same issue with my Cherokee for quite a while (no rear brakes), and will be attempting to sort it out soon. I question the need for the proportioning valve to be functional in a vehicle equipment with ABS. Seems a bit redundant.

Also, I was surprised to read posts claiming that the ABS doesn't work below 10mph. When I am moving at just idle speed on pure ice (maybe 3 to 5 mph), if I hit the brakes, my ABS kicks in even at that low speed.

94XJ4point04x4

buy a gauge and attach to bleeder valve on rear and read the pressure with the vehicle on and brake pedal to the floor, youll need another person, then repeat the same with the front and do the math. rears hardly do any work on a normal vehicle unless you have a height or load sensing proportioning valve which would send more pressure to the rear for towing. you can buy a guage at autozone or any other part store or go to harbor freight. if you're that concerned with your braking.

94XJ4point04x4

the vehicle can be on or off btw, on just means your booster will have vacuum and create more pressure. to test master cylinder engine off, use the guage on front and hold brake pedal down for a minute or so and see if brake pressure falls. if it falls, your master is bad.

mdtobe

I'm wondering if maybe some of you missed where the OP wrote this in his opening post:

"I put the rear end up on jack stands so I could test the brakes with out using the front end. I can pull my park brake and I can lock the rear wheels up where the engine can't really brake them loose, but if I use the foot brake, the engine doesn't have too much trouble turning the wheels."

Yes, we all know the rear brakes aren't as powerful as the front brakes. But unless the engine is putting out WAY more than stock power, when up on jack stands, the rear brakes should be able to easily overpower engine torque.

I have the same situation with my Jeep Cherokee - virtually no, or maybe completely no rear braking power through the hydraulics, even though I've got the drum shoes adjusted to where if they were any tighter, they would be dragging on the wheels all the time. If I recall correctly, even at idle I couldn't stop the rear wheels from rotating with the brake pedal. And my parking brake is nice and strong, so it's not an issue with the shoes or drums. I have a feeling my proportioning valve is the culprit. If it's not that, maybe my wheel cylinders are seized. When I get down to it, figure it out and fix it, I'll post the results here.

Best wishes,

John

94XJ4point04x4

insurance likes my "abs" too... they don't inspect it.

94XJ4point04x4

Me? I was answering his question on how to check proportioning valve and test master that he wrote on page 2.
Maybe you didn't read the comments.
Best wishes,
Dillan.

mdtobe

No, not you Dillan. I was referring to the comments made by respondents who told him it's normal for his rear brakes to be a lot weaker than his front brakes, and seemed to have missed the sentence in his first post where he wrote that the engine could easily overpower the brakes when the car was up on jack stands.

You can unruffle your feathers now!

And, Merry Christmas everybody!

kstopp

I think OP has figured his problem out, but I wanted to clear a few things up. The valve we are talking about is called a combination valve, it has several functions and parts including a metering valve, proportioning valve and a brake warning switch.

OP, are you talking about removing the seal on the metering valve? See photo below:




Source: http://www.mbmbrakeboosters.com/inde...ters&Itemid=10

Removing the seal on the metering valve, or the entire valve simply causes the front brakes to activate at the same time as the rear. This would be fine in a front and rear disc brake system.

jedijeb

OP probably won't reply since he originally posted in 2011