Diagnosing brake problem

[MartinQ]

I have a BMW 123d which has developed a vibration above 40 mph. I stopped to investigate and there was a burning smell and the front nearside wheel was noticeably radiating heat. I suspected a sticky caliper but I removed it and the piston moves freely. Still plenty of pad left too. I regreased the piston anyway and the back of the pads but the problem was still there. I think it must be brake related due to the heat generated and nothing looks broken or is rubbing.

Is it worth changing discs and pads and/or caliper? Anything I may have overlooked?

Thanks.

[DuraAce]

Master cylinder maintaining some hydraulic pressure? Is the opposing brake at the back hot?

[M32Guy]

Were the pads free enough in the carrier? Did you check the slides were moving back and forth freely too?

[MartinQ]

Now you mention it, I had trouble getting the outer pad back in - the protrusions(?) on the pad barely fitted in the guides on the caliper. Not sure why. But that may be the cause.

[E-bmw]

Some manufacturers are worse than others but I have to admit, I tend to get the angle grinder/file out when fitting pads to make sure they are free as they can be.

Another vote for checking/greasing the sliders too.

[tapkaJohnD]

Original problem was a "vibration above 40mph".
Nothing to do with the brakes, and the caliper appears normal, but it has overheated.

Look at the wheel, disc and stub axle. Is one causing a wobble, so that the disc contacts the pads when not braking?

Jack it up, spin the wheel. If this wasn't a BMW, you could do so with engine power. Any wobble?
Take off the wheel, look at the disc as it spins. Any wobble?
Look at the stub axle, does that wobble, even sightly?

John

[GreenV8S]

tapkaJohnD said:

Look at the wheel, disc and stub axle. Is one causing a wobble, so that the disc contacts the pads when not braking?

That would normally cause pad knock-back, rather than overheating, unless there was a problem preventing the pad from being knocked back - that's basically the same type of problem that would cause the brakes to overheat without the wobble.

[finlo]

I had this exact problem, caliper seemed ok so after weeks of theorizing about abs faults collapsed hoses etc any prolonged speed over 50 and the overheating would return.

Yep you've guessed it sticking caliper.

[tapkaJohnD]

I agree, Green, but the guy's okayed the caliper and piston(s).
Oh! Single piston? Jamming slide?

John

[helix402]

Sounds like a new caliper is needed.

[sassanach0]

if you have confirmed that the caliper is free and the pads are not sticking,then my next port of call would be the flexible pipe to that caliper.it has been known for the flexible to perish internally and act as a one way valve.

[McVities]

Is there a form of traction/stability control on this car?
Many cars will brake an individual wheel if the ECU registers something as out of control.
A problem with a sensor may be making the traction/stability control think something is afoot and causing excessive braking on the one wheel.

[SebringMan]

To me it sounds like one of two things is wrong;

-The slider pins are seized
-The pads are stuck in the carrier.

The former you may need to buy a slider pin kit or clean and regrease the bushes and pins. Whatever you do, do not use Copper grease ; I've seen so many seize up badly because of this. Use either Lithium grease or preferably red rubber grease.

For the pads I'd clean out where the pad contacts the carrier ; there is probably a buildup of rust and brake dust. Again, I prefer to use either nothing here or ceramic brake grease on the caliper ears.

The same thing happened on my Clio. Prior to changing them the brakes were horrendous:

https://www.pistonheads.com/gassing/topic.asp?h=0&...

[MartinQ]

Just to update this - I couldn't find a fault with anything in particular so took it to the local garage. It was something wrong with the caliper in the end. The piston was free moving but for some reason wasn't releasing fully. He dismantled it, inspected and rebuilt it but couldn't find anything and the problem remained. A new caliper and pads were installed and all is well again. No play in the axle, suspension etc so hopefully that's solved it.

Cheers for the advice all.