Never before seen brake issue...

So this isn't on a Tundra just to be clear, but I'm not a member on a Lexus or RAV4 forum, and brakes are fairly similar, so here is the deal...

This is my sister's 2007 Lexus RX350. I've been working on vehicles for 27 years now. I did it professionally for 5 years. I've done literally hundreds of brake jobs. It's been probably 20 years since anything on brakes has "stumped me". However, for the life of me, I can't figure this out.

So her rear brakes were squeaky. I yanked the tires, and sure enough the pads were worn down to the wear indicators. I yank the pads and the wear is perfectly even between the inner and outer pad. Even the driver side pads and passenger side pads are evenly worn. So no issues with the calipers with uneven wear.

I check the rotors and they are good. No hot spots, no discoloration, no grooves, and plenty of meat. So I rough them up with a 3M pad on a drill, and just install some new pads. The hardware on the Lexus looks to be in perfect condition and the hardware that came with the brakes (Wagner thermo-quiets) looked like trash compared to the factory hardware. So I cleaned up the factory hardware which honestly didn't need much, and left it.

I greased the caliper pins, put a very light/thin layer of grease on the back of the pads where the piston and caliper touches (this keeps the brakes from squeaking later from vibrations or slight shifting), and reinstalled everything. I used a couple drops of blue lock tite on the caliper bolts and retorqued them to factory specs.

Put the tires back on, and called it good. Everything went smoothly. I double checked everything as I don't like having to redo stuff.

So basically the only thing I did was replace the pads, rough up the rotors a little and grease the caliper pins.

So she drives it about 15 miles, comes back home, and you can smell burning pads. I check to make sure the emergency brake wasn't on (I never set it during the repair), and it wasn't on. I engaged it and disengaged it just to be sure it was off.

The back wheels were SIGNIFICANTLY hotter than the front wheels. The rotors were not glowing, but they sure as heck were hot.

So I let everything cool down for 3 hours. Then I drove her Lexus for 5 miles, not using the brakes at all. I just coasted when coming up to turns. At the end, I got out, touched the front rotor, it was barely warm. Then very quickly tapped the back rotor, and it literally burned my skin instantly. I spit on it and it sizzled. Again the brakes were not used at all.

The weird thing is, BOTH rear brakes are doing this. So it's not like it's a caliper hung up on one side. As I stated above in the beginning, both sides had even wear on the old pads. I do not believe it's the calipers hanging. Also the pistons on the calipers moved in very easily. We live in Texas and there is no rust on this vehicle.

Whatever is causing this is effecting BOTH back calipers at the same time.

Also when you put the car in drive, even in a gravel driveway on flat ground, it will roll while just idling. So the brakes are definitely not hanging up enough to keep it from idling forward or backward even on a gravel driveway.

Now unfortunately I don't have my infrared thermometer gun anymore (it broke) to check exactly how hot the rear is getting, but it just doesn't make sense to me that it's so much hotter than the front.

However the fact that both sides are doing this, and the car still rolls forward at an idle on flat ground, doesn't make sense to me. I've seen hanging calipers, but never two that are perfectly hanging even. Also typically the car won't move at idle, almost like the emergency brake is set, when they hang.

Anyways, I've been racking my brain all afternoon and night and I'm stumped. I can't find anything online about it that makes sense.

It's been many years since anything had stumped me like this. If anyone has any ideas, please let me know. It will be greatly appreciated.

 

This happened to me. To be clear, it was on my Chevy, not my Tundra. I was on the highway and the car felt sluggish. I pulled over and the rotor was glowing bright red. I had to get the calipers rebuilt and flush with new brake fluid.

The issue? My brake lines seized. I did not believe that at first. But the hose looked fine. Inside, the rubber brake hose had collapsed. Seems the car’s power brakes were powerful enough to push fluid in, but it couldn’t drain out.

My recommendation would be due to the car’s age is to replace the rubber brake lines, obviously bleed the system with fresh brake fluid. Check the caliper rubber for signs of heat damage.

 

So I've been spending some time thinking about this, and the one thing I didn't do on this brake change, that I've always done on every other vehicle, is open the master cylinder cap or loosen it while pushing the caliper pistons back in. Now I'm not an engineer, but pushing those pistons back in, is going to force the brake fluid to go somewhere. If the cap wasn't open, that would have created a pressure in the brake lines and master cylinder correct? Could that pressure be slightly pushing on the caliper pistons? That's the only thing I changed from every other brake job I've done. I was sick while swapping her pads and I just forgot while being under the weather. I'm wondering now if that caused this. I just don't know how those master cylinders work. Like if they are always pressurized, or if the cap has a vent, or if the piston is always sealed or only when you are pressing the brake, etc. There are just too many variables. I tried "burping" the master cylinder by opening up the cap, and thinking maybe there would be some pressure bleed off.

However I just finished driving it again, and the rear is still starting to overheat after just a 15 mile trip with almost no braking. The pads are definitely very slightly "hanging" on both sides of the rear. What's crazy though is they are hanging so lightly that the vehicle will still idle while in drive and move. So it's very minor.

My gut tells me I should bleed the rear brakes to try to alleviate some pressure that maybe is stuck in the rear section behind the proportioning valve maybe?

While I am in there I can inspect the rubber hoses and look for a collapsed brake hose as well. If that doesn't work, I'm at a total loss on this, which is absolutely insane because this is literally probably the three hundredth brake job I've ever done, and I've never seen or heard of anything like this. I even contacted some buddies who were professional mechanics with me back in the day and they are also at a loss. Just swapping some pads out should not have caused this.

 

I would also check the pads to make sure they can move freely in the hardware. I had something similar happen to my wife’s Corolla. I had to grind the pad backing plate down a little so they wouldn’t stick.

 

When compressing the pistons on the brake caliper on a car equipped with ABS, I always crack the bleeder screw so that the always contaminated fluid in the caliper gets bled out of the caliper and not back into the brake lines, and also not sending any pressure or fluid back towards the ABS module. It’s possible the brake pistons in the rear calipers are both corroded as well. New pads requires pushing them further back into the bore where corrosion present that previously wasn’t making contact with the bore, now is.

 

Regarding brake bleeding brakes, I learned my lesson a long time ago. Brake fluid does absorb moisture, which may rot the inside of my calipers. I have been using ATE Original TYP 200 Racing Quality DOT 4 Brake Fluid in all my cars. It is what I need when I track my Chevy, so I use it on all my cars. Never had an issue with it.

BTW: Dot 4 absorbs moisture at a slower rate than Dot 3, however, the boiling point of Dot 4 will drop a greater amount with the same % moisture than it will with Dot 3.

 

This is a pretty common thing that happens but it's usually on higher mileage or older vehicles but I'd definitely consider replacing those hoses if none of your other options get you anywhere, they aren't usually very expensive @Slp82

Usually isn't much to see unless you cut the hose in half, at that point you're replacing anyway.