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Thread ID: 83040	2007-09-17 04:59:00	car brake imbalance	lance4k (4644)	PC World Chat

Post ID	Timestamp	Content	User
591931	2007-09-17 11:03:00	Dosent the brake proportioning valve just regulate the pressure to the rear brakes to stop them locking up? They are fail safe if it fails you just get equal braking front and rear A brake imbalance would be between left and right. tedheath	tedheath (537)
591932	2007-09-17 11:33:00	Proportioning valves distribute the brake pressure equally to both sides but differentiates between the front & rear brakes. The front brakes do approx 75% of the vehicles braking Load limiting valves adjust the amount of pressure applied to the rear brakes. The idea is to stop the rears from locking up when only light pressure is applied to the pedal & with no load in the rear of the vehicle. These are normally fitted to vans & utes & are sometimes miles out of adjustment.	Phil B (648)
591933	2007-09-17 17:33:00	OK . . . a good night's sleep under my belt . . so here we go . . . . . . . . . KISS . . or "keep it simply, stupid!" is the motto to keep in mind here . First, start by understanding your system . There are a few different things that may not apply to your question . . . a year/make/model would be helpful, but minus that, I can start with "Hydraulic Brakes, 1A" . There are very few modern cars with problems in brake imbalance . If you mean front-to-rear imbalance, then you are not fully understanding that MOST braking is done by the front brakes . Some times the difference is 60-40, other times 75-25 . So there's a lot to know in the system and it's original design . Almost all front wheel drive vehicles have higher front-to-rear biasing because the front is where most of the weight is and they can apply the front brakes harder and the weight keeps the tires from slipping . The way this bias is set varies from manufacturer to manufacturer, but it is usually accomplished by a combination of . . . brake TYPES (disk front/ drum rear), size of brake hydraulic cylinders or piston area, mechanical design (dual leading shoes: called the Lockheed style), frictional co-efficiency variables (different brake material front-to-rear), proportioning valves, COMBO valves, biasing valves, delay valves, electronic controls (usually with split or dual master cylinders, sometimes remote mounted for each system . ) master cylinder piston sizes . . . etc . So you can see that there's a lot to consider . Typically, we see the more "hi-tech" types of systems in up-cost vehicles and not usually common in econo boxes . If all things are near or at normal in terms of wear, friction material coefficient (organic/semi-metallic/ceramic/hybrids or any or all of the aforementioned), that the adjustments are all pretty close to factory specs (front disk brakes on MOST vehicles are NOT adjustable yet the rears are in many cases) and the master cylinder is doing it's part, and the proportioning valve is not stuck . . . then all is well . Let's talk about proportioning valves for the nonce . They rarely fail or act up . If they have had to work with nasty fluid . . and that's a different story for the end of this article . . . they CAN fail . . . but that is very very very rare . There IS a valve that is usually mounted on the rear suspension somewhere that detects the load in weight in the back of the vehicle that then allows more brake biasing to be moved rearward . This is a "load-compensating" proportioning valve and must not be confused with a typical valve that just lies there keeping the same-o values all the time in spite of the load and or cargo changes in the vehicle . Put a really fat person in the rear seat and the dynamic valve changes the bias whereas a non-dynamic valve cannot . So much for front to rear designed-in biasing . On to defective front-to-rear biasing problems . IF you have not used your parking brake (most front wheel drive EXCEPT SUBARU here), then your rear brakes IF THEY ARE DISK OR DRUM, are likely out of adjustment . Here's where it gets a little tricky . You can adjust the rear brakes on either type of set-up . . . but it is done by strongly setting the parking brake each and every time you park the car . What happens in REAR DISK applications is that the calipers have a "ramp/screw" device build into the inboard pistons that compensate for the pad wear by rotating the screw and keeping the piston more and more fully extended as the pad wears . If this is not adjusted by using it or it's not working, then you lose the front-to-rear biasing that the factory designed into the system . The reason: The parking brake is mechanical and the service or driving brakes are hydraulic . Addendum: SUBARU uses the front brakes as the parking brakes . . very different from MOST other vehicles . They operate like the rear brakes in other all-disk brake systems . This system is rare for the average shadetree mechanic to run it to, but it exists . . so be careful about this point . Setting up the rear disk brakes with parking brakes is done by turning the rear piston back to "home" at the time of replacing the pads . You have to screw the pistons back in where they can start anew with the new thicker pads . Pistons with this capacity will show some sort of design on the face of the piston . . . notches, holes, some sort of way to attach a tool to rotate the piston back to the starting point . You don't have to have the special tool . . but it helps a lot . They screw in, right handed threads . NOTICE: If the external boots are broken, missing or ripped, it's a good idea to rebuild or replace the calipers . This broken rubber part will allow rood dirt and water/debris into the internal seal area . . although not into the fluid, and corrode the piston likely making it seize or stick during normal use . The front pads are very simple . . . just use a pair of Channel-loks and squeeze the pistons back in to home . Don't have the opposite side disassembled at the same time as you will force the piston back into the bore on one side whilst the piston on the other side will pop out of the caliper and make a general mess of your day . Pay attention to the force required to reseat the piston; if it won't go, try to tap into the center of the piston firmly with a wooden hammer handle to get it started . It is not unusual to have to make it move first with some force to get things going . If you cannot seat the piston back to "home", then there may be too much debris in the cylinder where the piston is and it is going to need to be rebuilt or replaced . Calipers CAN be rebuilt . . they were made by a human and can therefor be repaired by one too . "Home" is usually where the piston is level to the top of the caliper cylinder face . If it sticks up, it is usually not home and will not allow new pads to be installed . I never turn (machine) my rotors on my own vehicles . . I know . . . this may fly in the face of most people who insist that any grooves or scores are dangerous . . . . but they are not . . . the pad will comply with minor grooves or disfigurements with ease and will actually improve the swept area and increase the braking effect . (FOOTNOTE: General Motors in the early days of disc brakes machined INTO the rotors a single deep circumferential groove to keep the pads in place once they established a wear pattern to keep the water out and the noise down . ) . . so . . . grooves and wear are not bothersome but metal-to-metal wear IS! If you machine the rotors, then you take away a lot of the heat sink capacity and the rotors will warp much more easily next time . Side-to-side imbalance can almost always be attributed to a sticky piston in the caliper on one side or the other . If you have one sticky caliper, then you MUST change out both calipers on the same axle, is: both front calipers or both rear calipers . Replacing one on one side will usually then cause an imbalance in the system as one is brand-new spanking clean and the other has seen some contamination . Be careful here . . . I have found mismatched calipers from the factory some times that drove me crazy . Be aware thought, that this type of imbalance can also be caused by "radial pull" or the fact that a tire is generating a lot of rolling resistance from being rotated to the wrong position . . . or the tire may just be bad or defective anyway . Brake hoses are not likely to cause imbalance . . but it can happen . They usually show some sort of physical damage to the outer covering if they have had physical blows or a pedestrian got stuck under the car at some time or another . Darned pedestrians grab at anything as they go underneath! Let's talk about the master cylinder here now . First, sit in the car and press your foot with very little weight on the brake pedal and hold it there . . maybe even jiggle or tap dance on it a little and see if the pedal sinks or slowly drops lower after a while . This may take a minute or two . . but try it to see if we are not dealing with a bad master cylinder here first . You SHOULD have had a brake light lit on the dash if you have a defective or broken master cylinder . . but some times a butthead owner will take the bulb outta the socket that normally would signify that there is a casualty in the system . That light is not just for looks . . it is a warning device . You can test the bulb now . . sitting in the same seat . . . just turn the ignition to ON, not bothering to actually start the engine and see if the light comes on and then goes off . . . or stays on . . . . or never comes on at all . Try setting the parking brake to see if the light . . which is usually the same one . . comes on then . If not . . find out why! This light is multi-functional . . it tells if the p-brake is applied, there is a failure in the pressure differential valve (proportioning-combo valve if it has one) or if the master cylinder is bad or in some cases if there is insufficient vacuum in the power brake reservoir . Ford uses a "BOO" switch for this . . . there may be others with the same idea . :yuck: The "Parking brake" is NOT and "EMERGENCY brake!!!! On modern vehicles, there is no emergency brake . Get over it . Air in the system is not usually a problem . . but it indicates a problem somewhere . Air should never be in the system to let it work correctly . . . . again, the brake warning lamp should tell you this has happened . It can tell if the system has air in it by the pressure difference in the system . Air will also not cause an imbalance in side-to-side dynamics but WILL cause grief in front-to-rear braking . Both front or both rear brakes with the air in them will act strange, as the rules of hydraulics say that the pressure in the line will be equal in any lines that are common to it . The front and rear brakes are separate, not the left and right ones . EXCEPTION TO THIS RULE HERE: There were some UK cars that tried this system,,and a few French cars too . They are mostly gone now . . but they might still exist in Upsidedown Land . I seem to remember the Austin America and the Citroen and maybe a few Peugeots as examples of this silly system . Here's a good rule: NEVER OPEN UP THE MASTER CYLINDER TO REFILL THE FLUID! Why not? Brake fluid is hydrothropic . . . . it likes water and draws it from the air if the system is opened and topped up all the time . This is why modern master cylinders are semi-see through . . to tell when the fluid is actually too low without actually opening the cylinder . There is also a level senser too and it will also turn on the BRAKE light on the dash if it gets too low anyway . The reason for not opening the master fluid reservoir is kinda obtuse, but here goes: Since water is a very bad thing to get into the system, the manufacturers don't want you to open it to the atmosphere where water is in the air . . . and water will destroy via contamination and corrosion, the ABS valves . We don't care, except in the very long run, about the cylinders or calipers or such . . . it's the ABS system that gets it in the shorts if you let water into it . When performing a brake job (some cars require a bi-directional scanner to tell the ABS system that it must ignore any symptoms that are about to happen) we usually just force (gently) the fluid back into the master cylinder from whence it came from the then extended caliper pistons, so we don't contaminate the system . . especially the very $$$ ABS part! Bleeding the system is usually NOT the best thing to do . . . . . but I find I like to do so once in a while . . depends on the system and the time factor involved and if I am searching for a secondary problem . To do so, and if you don't have a bi-directional scanner, just open the bleeder nipples, one at a time, at the cylinders/calipers and allow the fluid to gravity flow until you see clear fluid or not bubbles at all . Remember to gently tap on the caliper(s)/cylinder(s) with a wooden hammer handle to shake loose any microbubbles that might cling to the walls of the cylinders at the same time . Keep the master reservoir full at all times . . I use a smaller fluid bottle and just invert it into the filler cap area and let the fluid gurgle into the reservoir as it is used . A little air is not harmful or noticeable and will work it's way outta the system eventually anyway . OK . . some basic rules . . . . . Make sure the warning light(s) for the ABS, parking brake operate when the key first gets turned on . . . provided the p-brake is applied, of course . Make sure that you actually have fluid in the system . Never open the master cylinder to add fluid . . if you need fluid, you need a brake inspection anyway! Perform a visual inspection of the rotor(s), drum(s) and lines/hoses . KISS! (Keep It Simple, Stupid!) is a very good rule to obey! Sorry for the length again . . . it is hard to compress this info into MP3 format . .	SurferJoe46 (51)
591934	2007-09-17 17:45:00	Sorry Joe . My cheap and cheerful idea works . Been there, done that . It's only if there're much more complicated problems that stop the brakes balancing in testable results, then . . . perhaps go to your [unread lengths] to try . My idea has worked in the past . :thumbs:	Greg (193)
591935	2007-09-17 18:08:00	Sorry Joe . My cheap and cheerful idea works . Been there, done that . It's only if there're much more complicated problems that stop the brakes balancing in testable results, then . . . perhaps go to your [unread lengths] to try . My idea has worked in the past . :thumbs: What type of test are they doing there to detect that sortta problem? We have dynos that are single roller and dual roller (for side-to-side consistency) during our smog tests . . but they don 't test for brake imbalance . We never had to test front wheel drives b4, but when they got popular and became the greater market . . we had to use the dynos with the right and left sides separated so the cars would not jump the rollers or try to go left or right and off the dyno . But our dynos are really non-powered so we don't get to see what youse guys see . We DO pull a "brake/lamp/smog/weight" test on vehicles coming out of recycled and those that fell out of registration as a punishment for those who do so, but they are really an exception . The brake test requires a full tear down and measurements taken and conditions noted; The lamp requires alignment, candlepower and DOT standards that dictate the legally allowed types of bulbs, lenses and height and designs on the vehicle . Smog tests are getting stricter and even older vehicles are now required to pass standards that they did not have to pass when they were new . Some of the older (1966-1985) will now be tested for modifications, removals of originally installed devices and engine transplants . Older vehicles with newer power will have to comply with the "year of the engine" requirements and also pass the "MMD" specs (Modified/Missing/Defective) . Put a 2005 Chevy engine into a 1966 pickup, and it has to have the whole computer system and all emission related devices as were on the original engine/vehicle combination installed . Weight is just that . . . and it is required on a certified scale . . . it is usually for trucks (lorries) that are suspected of overweight modifications or those that have been modified and need a new cert . That's about the all of it . . . but imbalances are not tested . I still don't recommend a "burn-in" to correct something that is wrong . . it's just not good mechanics . There has to be a cause . . even if the owner parks the vehicle with two wheels in a creek . . but there has to be a cause that should be addressed .	SurferJoe46 (51)
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