Brake pad wear issue

Just did the brakes on my 06 Scion Xa and noticed that the passenger side inner pad (pad on the piston side) was noticeably less worn than the outer side pad. The two pads on the driver side showed even wear and matched the wear on the passenger outer pad. So, it appears the passenger piston side pad did not wear nearly as much as the other three pads. What could the cause be? I did not see any issues with the hardware, the pistons retracted easily on both sides. the sliders were definitely not frozen but probably were not sliding as well as when cleaned and re-greased. This is a pretty simple single piston system. Any thoughts?

Probably the brake lube on the sliding parts was missing or contaminated, as you suggest. I presume you didn’t notice any brake pull. Check the new pads in 5,000 miles or so, see if the problem is repeating itself. On my Corolla if I remove the wheel there’s slots at the front of the caliper, so I can see, and with the aid of measuring calipers, measure the pad wear without having to disassemble the brakes. Might be the same with your Scion.

I had an old Tercel that wore the right rear brake twice as fast as the left, and I never did figure it out.

Your caliper is sticking.

My original thought was that the slider was sticking too. There was no brake pull or other performance indication. What i don’t understand is why the wear on the sticking pad would be the same amount as both the pads on the other side. It’s like the supposedly sticking pad is wearing normal while the piston side pad is the one that is off somehow. I will check it out at my next tire rotation in 3K and maybe relube the sliders again.

I guess the “non-wearing” pad could have been made of a different compound, i.e., someone installed non-matching pads.

A sticky caliper will not necessarily cause pulling. It can, however, cause the pads to not press symmetrically on the disc, and can cause a pad to drag slightly, either or both of which will cause one pad to wear out disproportionate to the other.

A few months ago I replaced my left rear caliper on my '05 tC for exactly the same condition. What you’re experiencing is not at all unusual for a car approaching its first decade.

I don’t know what you are calling sliders, but Toyota’s float on a pair of bushings, at least the older ones did, the newer ones may use pins instead. In either case, yours are sticking so the outer pad is not releasing completely on the passenger side. This will not cause pulling when braking but there will be a little more drag on the right side after the brakes are released, but it doesn’t last long, just long enough to take a little more material off the outer pad.

insightful: Nope definitely same material and matching pads.

the same mountain bike: i can definitely see that a sticky caliper would cause the outer pad to wear faster. The thing i don’t get is why this “unique” inner pad on the pass side shows like 50% less wear than the other three pads including the driver inner pad. I would expect the two inner pads to have similar wear. I guess i could have had both calipers sticking AND the driver side piston not retracting properly but that seems a bit much. Three pads with similar wear and one significantly less worn. It’s a stumper!

My car the inners were worn to the nubs but outers still good. @85k pads rotors fuid etc. redone. They were getting soft 80k later, my guy said the pads were ok but rotors pitted as heck. I said at 80k on them let’s just go fresh. No explanation for inners wearing faster than outers.

It isn’t a stumper at all. It’s a simple sticky caliper. Which pad wears depends on how it sticks.

The concept is that the calipers operate like a hydraulic C-clamp that “floats” on pins. When the hydraulic pressure is applied, small rubber “square O-rings” become distended from the pressure pushing the piston further out of the caliper. When t he pressure is eliminated and everything relaxes, the “square O-rings” go back to their relaxed state and in doing so pull the pistons back just enough to raise the pads away from the discs. The caliper being “floating”, it relaxes evenly. If it’s sticking such that it continues to drag the pad on the inside, the inside pad wears thin. If it’s sticking such that the outside pad is dragging, well, you get the idea.

I wonder if some kind of brake temperature measurement after a few miles driving could help pinpoint the problem? If anything is dragging, the temperature of that component would increase more than the non-dragging components.

An infrared thermometer will confirm that it’s dragging, but won’t tell you why.
You can buy an inexpensive one for around $50, and IMHO it’s worth every penny. I use mine all the time, not only for the car but also to determine where I might want to add more insulation to the house, if I have a leak around a window seal, and all sort of other things. For the car, you can also use it to evaluate cooling systems and countless other tests.

I confirmed that I had a dragging pad that way.

If it's sticking such that it continues to drag the pad on the inside, the inside pad wears thin. If it's sticking such that the outside pad is dragging, well, you get the idea.

But this doesn’t explain how the one inner pad out of the four front pads was wearing at half the rate.

If it’s sticking such that it continues to drag the pad on the inside, the inside pad wears thin. If it’s sticking such that the outside pad is dragging, well, you get the idea.

But this doesn’t explain how the one inner pad out of the four front pads was wearing at half the rate.

THIS is exactly why i am stumped. Are three pads sticking and one is not? seems unlikely. Is the one side fine and the “issue” side experiencing a stuck outer pad and a piston that is not pushing the inner pad often/enough for it to wear? also seems unlikely since i would expect to feel something in the braking if one piston was not functioning properly. But who knows!

The caliper containing the pad that isn’t wearing is sticking such that the pad that isn’t wearing isn’t pressing on the disc with the same pressure… it’s sticking FROM applying force. The caliper on the other side is totally unrelated. It’s distributing the pressure applied to it without regard to the sticking caliper on the other side.

The Master Cylinder contains two pistons. One provides force to the right front and left rear calipers. The other provides force to the left front and right rear caliper. It’s called “diagonal braking”. It’s designed that way so that if you spring a leak, you’ll still have half your system and it’ll brake in a somewhat balanced manner. Each caliper, if it’s floating freely as it should be, will distribute the forces equally between the inside pad and the outside pad. If one of the pads is sticking, the force will be the same on the piston behind each pad… but only one will move. Since the force being applied to the pistons on the opposing caliper are the same as the forces being applied to the pistons on the sticky caliper, the three pads contacting the discs will wear somewhat equally, but the stuck pad won’t.

Think of it this way. Pressure throughout all surfaces within the system, in both sides (assuming the MC is working properly) will always equalize. The pressure on all the caliper pistons will always equalize. That’s the way fluids work. You have four calipers, one on each corner. Each caliper is closed like a clamp, by its piston, around the rotor. The caliper piston is what closes the clamp. If one side of one clamp is stuck, the other three will all close with pressure equal to the surface area in inches times the PSI applied. One stuck pad does not necessarily affect the pressure with which the others are being pressed against the disc.

So if i understand you correctly, you are suggesting that the less worn inner pad is somehow sticking/binding where it sits in the caliper such that when the piston applies force to it, it is not being pushed into contact (much) with the rotor, and is providing the leverage for the piston to pull in the pad on the outer side?

Imagine you have a piece of 2 x 4 wood freely floating between the two jaws of a vice. You tighten the vice, both sides of the 2 x 4 would feel the same pressure, right? But if you put some kind of a spacer on one side of the 2 x 4 that prevented that side from moving up and touching the jaw, that side would experience no pressure at all.

Sort of, but not quite. If the pressure in the line is 50 psi, it’ll be 50 psi on both pads, but rather than the caliper floating and allowing equalizing of the pressure on both pads against the disc, the caliper will be sticking and the 50 psi will only be pressing on one side of the caliper rather than both. The pressure inside the caliper system will be equalized at 50 psi, but the stuck caliper will prevent it from moving one of the pads into the disc freely.

As regards the caliper on the opposite side, if you’re applying 50 psi to the MC, the MC piston operating that side will also be applying 50psi to that caliper as well, but since it’s free floating it’ll allow the caliper assembly to float and apply that 50 psi to both side of the caliper. So you’d have three of four pads pressing the rotors at 50 psi and one not.

The pad opposite the functioning pad doesn’t really provide leverage. It’s simply a matter of the pressure equalizing throughout the caliper assembly and the caliper assembly itself stuck and not free floating, resulting in only one pad contacting the rotor.

NOTE: your brakes operate with pressure far exceeding 50 psi, but I picked that number out of the air to illustrate.

I wanted to add a point about hydraulics that might help. If a given pressure, say 50 psi, is applied to a hydraulic system (or any fluid system), that pressure will always equalize through out the system. The pressure on all surfaces inside the system will become 50 psi. Pressure of a fluid will always equalize. If there are two sides of a component that are supposed to cause the pressure to clamp two pads on a disc, and one side of that system is stuck, its pressure does not move over to the unstuck part and create 100 psi there. The pressure inside the unstuck part remains at 50 psi no matter what the stuck side does. It’s simply a question of the pressure equalizing in the system.

The bottom line is that your caliper is sticking. To fix the vehicle, you need to replace the caliper.