Replacement Front Brake Pads Don't Fit Easily Into Caliper
Posted 04 August 2009 - 08:13 PM
Today I purchased new front disks, pads, and shims from my Toyota dealer for my 2003 Matrix (Corolla), as this was to be the first time the pads or disks were ever touched on this car already with 70,000 miles on it.
While changing these parts, the same problem happened today that happened when I did the front brakes of a Nissan Sentra 5 years ago.
When it comes time to snap the new pads into the pad support plates (that pair of shiney clips which are pressed into the caliper torque plate), the pads don't want to slide in, the pad's tangs are slightly too large.
As with the Sentra pads, I used my bench grinder to remove the green paint on the tangs of the pads plus a few extra thousands of an inch of metal, and they now fit.
So why would stock pads not fit into 7 year old calipers?
My guess is that my unpainted torque plate rusted all over its surface and that extra corrosion under the stainless steel clips pushed the clips (support plates) closer together and pinched the tangs on the new pads too much to enable a clean fit.
If I understand the way disk brakes work, it is essential for the pads to "float" within the caliper so that when there is no brake pressure applied, the disk can push back the pads away from the surface of the disk by a few thousands of an inch, enough for the disk to spin without contacting the pads. If the pads are pinched within the clips, then the disk will not be able to push the pads away and the pads end up dragging lightly even if there is no hydraulic pressure in the system.
An alternate to grinding the new pads would have been for me to clean out the old corrosion under the clips and install new clips and then hope that everything would fit.
Has any other Corolla owner experienced the same problem of a too tight fit when doing their own front brake job?
Is it necessary to apply a lubricant between the clips and the tangs of the pads?
Posted 04 August 2009 - 08:20 PM
Posted 04 August 2009 - 08:33 PM
always clean the caliper hardware or replace it, sometimes the paint is a little too thick on the pads and must be removed. all moving parts of the caliper should be lubed with silicone grease.
Thanks for your reply.
It's reassuring to learn from your response that the thickness of the paint can indeed create problems.
My caliper pins are well lubricated with silicone grease, but there is none between the tangs (ears) of the pad and the stainless steel clips.
I'll pick up new clips tomorrow and add lubricant when I install them.
Posted 05 August 2009 - 09:02 AM
Posted 05 August 2009 - 01:43 PM
The ones on my Matrix were the same way - was just the paint on the backing pad of the pad and corrosion working its way underneath those clips - razor blade to shave off the paint and a wirebrush to touch up the mating surface between the torque plate and the SS pad clips was all that I needed. They should be held in there pretty snug, on my Corolla - I had a set that the ears were the right size, but the clips were really thin - they slipped in pretty easily. Ended up, they were too loose and frequently squealed under light braking. Replaced the shims with a new set - the "ears" were pretty tight in the clips, but not so tight that I couldn't remove the pad by hand - no more squeaking. As for lubing those ears - I've never had to do that on any of my cars. Maybe if I planning to store the car outside for an extended time - might do that to keep corrosion from freezing the pad to clips - but for a daily driver, just lubricating the caliper pins is plenty.
Thanks fishexpo101 for your update.
I went back to the dealership this morning in search of some brand new SS pad clips and I was told that they never need to be replaced, only cleaned. They told me to snap out the clips and clean the rusted surface of the torque plate under the clips not only with a wire brush but also with a file. Fortunately I had a small triangular file which was perfect for getting into the tight grooves located under the clips.
I was also told to put silicone on the clips and on the ears of the pads so that the places on the pad ears where I had ground off the paint would be protected from corrosion.
I did some extra grinding of the ears to get everything to slide smoothly. I recall when I disassembled my 7 year old pads yesterday, the pad ears were so corroded that I needed a screw driver and hammer to knock out the pads from the torque plate.
Today I did get confused when it was time to assemble the outer pad and its two shims. The outer shim can only be mounted one way, but the inner shim has two bent tangs (one at each end), and I am not sure if the tang bend faces inward or outward. If it faces inward, then that would create a flex in the inner shim. The inner shim of the inner pad has no bent tang, so there is no issue. I chose to have the bent tang point outward which is opposite to what the diagram in the Toyota manual suggests.
If I have kept the shims from my old pads in the proper order, then it would appear that one brake had the bent tang pointing inward and the other brake had it pointing outward.
I have been doing some research with Google to find out how the pads are kept from touching the discs when the brakes are not applied. It seems that the rubber boot of the piston (roll back seal) is designed to slightly push the piston back into its cylinder whenever the hydraulic pressure is released. And that rubber boot works in a self adjusting manner so that as the pads wear thin and the piston needs to be further extended in order to compensate for the up to 10 mm. loss in combined pad thickness, then the amount that the piston retracts remains constant over the lifetime.
However even if the piston retracts somewhat, and the caliper pins and sleaves are well lubricated, I still don't know if enough of a gap will be created between the disc surfaces and the pad surfaces to eliminate any contact whatsoever.
I came across a patent design for another brake manufacturer (no. 6,378,665) which calls for an addition spring to force the pads away from the disc surface whenever the brake pressure is released, this in order to supplement the "roll back seal" in the piston.
Not knowing exactly how Toyota gets the pads to back away from the disc surface, I spent a few hours sizing the pad ears so that the pads would slide easily in the ss clips just in case that is Toyota's secret way of achieving pad retraction.
On a side note, this morning upon taking my car for its first test drive since changing the pads and discs last night, the check engine light came on and has stayed on for at least 5 engine restarts.
I know there is a sensor in the master cylinder resevoir and I wonder if that sensor got affected by my brake fluid displacement and is triggering an engine code?
Edited by montreal, 05 August 2009 - 01:50 PM.
Posted 05 August 2009 - 05:47 PM
As far as the mechanism for pads releasing cleanly from the rotor face - a combination of very slight processes that have tangible, commulative affest. True, that dust seal will pull back the piston slightly, coupled with the release of hydraulic pressure - once you release the brake pedal. There are also some aerodynamic forces present in a spinning disc configuration that will help "push" the pads away. You'll also have very slight dimensional variations during operation of the brake system (pads, brakefluid, metal surfaces, etc. will expand slightly, the release of line pressure will help drive those operating temps down significantly - and they will contract slightly). Vibrations/disc "wobble" are also prone in this system as well - they help "pry" the pads away from the rotor at speed. Even then, the may make very slight contact with the face of the rotor, which is perfectly normal for disc brake systems.
The biggest contributors are the dust seal actively resisting significant deformation, aero effects of a spinning rotor, and vibration/wobbles - being the primary mechanisms for pad release. There are several papers that you can find for free online - (SAE, IEEE, etc.) that speak of those physical mechanisms in greater detail if you are interested. Also why when some owners have upgraded to drilled/slotted rotors - they find that the pads do not cleanly release (aero mechanism has been altered) - depending on the caliper/rotor design, this effect can be quite pronounced.
I've seen similar applications for springs to supplement the release of the pads from the rotor. Was a big deal on mechanical systems and on drum brakes - but for good reason. Those systems were not as self-adjusting or had no other avenue to put any significant distance between the pads and rotors.
Posted 05 August 2009 - 07:19 PM
Pretty hard to believe that displacing the fluid in the reservoir in that manner would tripper a CEL. I would think it would trigger the body computer (BRAKE lamp or even ABS lamp, if you have ABS). Did you get a chance to pull the code yet to see what it was?..............There are also some aerodynamic forces present in a spinning disc configuration that will help "push" the pads away. .......... Vibrations/disc "wobble" are also prone in this system as well - they help "pry" the pads away from the rotor at speed. Even then, the may make very slight contact with the face of the rotor, which is perfectly normal for disc brake systems.
Thanks for the insight. I have always believed that the pads fly above the surface of the discs in the same way that the heads on a computer hard drive fly above the magnetic surface. Ironically, Toyota pads have a wing like profile, i.e. there is a ramp to channel air towards the rotor surface.
As well, there is always a microscopic wobble (runout) in the rotors and the high spot does indeed push the pads back enough so that there is minimal contact.
But for all these behaviors to work, the caliper must be able to open slightly wider when the pressure is released and the caliper pins have to be well lubricated in order for the caliper to float to a position whereby both inner and outer pads experience similar amounts of retraction.
Coming back to the pad ss clips. Since the torque plate is bolted to the car and does not move relative to the plane of the rotor, the SS clips which support the pads are likewise held at a fixed distance relative to the plane of the rotor. So whether the brakes are activated or not, whenever there is rotor wobble which wants to push against the pads, if the pads are to be successfully retracted from the rotor surface by the wobble effect, then the pads will need to slide easily in the SS clips.
This is why I went to so much effort to ensure that the ears of my pads were well fitted to the clips to allow the pads to slide lateraly.
Independant of the caliper's ability to float freely, if one or both pads are rusted solid to their ss clip and each clip is located a fixed distance away from the rotor surface, then the pad(s) is going to be glanced by the wobbling rotor on each revolution rather than be completely free of contact.
As for the CEL warning light, I hope my jackstand didn't hit the wire of the rear O2 sensor which is located not far away from the front right wheel well. If it doesn't clear in a few days, I'll have the ECU scanned.
Edited by montreal, 05 August 2009 - 07:23 PM.
Posted 05 August 2009 - 09:19 PM
Posted 06 August 2009 - 05:35 AM
the 7th gen corolla and some other toyots have 2 V shaped steel springs that help push the pads back very slightly, i dont see it anymore on new toyotas but they were very common in the 80's to mid-late 90's toyota brakes.
I don't know how many vintage and recent designs use a steel spring to retract the disc pads, but one limitation of this approach pointed out in the patent no. 6378665 (assigned to brake manufacturer Kelsey-Hayes) is that as the Toyota pads wear down over time starting at 11 mm. of thickness each and ending up with as little as 1 mm. each, the V shaped spring exerts more tension against used pads as it compresses that extra 20 mm. (3/4 inch).
That extra tension indirectly forces the caliper piston further back into the cylinder and your foot has to travel further down each time you press the brake pedal. The Kelsey-Hayes design proposes a U shaped spring which has a non-linear increase in tension as the spring displacement is increased, thus allowing the roll-back seal to operate identically with both new and used pads.
9th generation Corolla disc brakes do not have any retraction springs so the brake pedal displacement over time has to be managed by resorting to other design characteristics, no doubt some are noted in post 6 by fishexpo101.
In the absence of steel springs, to me it seems obvious that the current Toyota design depends on all brake parts sliding perfectly in spite of corrosion building up over time on the unpainted torsion bar. And that calls for a proper fit of pads when new and annual maintenance to see that they are staying that way. I found it discouraging that I had to take a few extra hours of my time to tailor the size of the new pad ears to work well withing ss clips that were attached to a 7 year old corroded torsion plate.
Thanks for your comment.
Posted 10 August 2009 - 06:49 PM
I looked at 4 Corollas and Matrixes in the parking lot of the dealer and 3 out of 4 had the tang pointing out and the other one may have been an after market shim without a bent tang. The service counter confirmed this reality.
Concerning my CEL which occured during my road test following my brake job, it is a P0446 and that is for the EVAP system and has nothing to do with the brake system. Just a coincidence, scary at first, least my brake job indirectly have a negative effect on my engine computer.
Edited by montreal, 10 August 2009 - 06:53 PM.
Posted 25 August 2009 - 02:33 AM
There ca be some "doctoring" required, such as removing some paint or lubing a portion of them.
Good to hear that your brakes work.
Now you have to diagnose and fix your CEL illumination problem.
Check out some of the earlier posts on the same subject for a solution - good luck.