Re: Can anyone check whether brake pistons move freely?

remove the brake pads and refit the caliper, it should now be possible to slide the caliper (if it is a sliding caliper) using hand pressure.

usually 4 piston calipers are not sliding type.

with the pistons pushed out a bit (by carefully pressing the brake pedal) it should be possible with strong hand pressure to push each and every piston back in to the caliper. A short lever may be used if you are weak or it is difficult to get a purchase. It is usually easy to tell if any piston is sticking, it will be obvious by the greater amount of pressure needed.

while the pistons are sticking out a bit you can check for rust or damage to the polished outside surface of the piston.

brake shudder is almost invariably caused by warped discs

mrcheerful

That sounds like complete and utter bollocks to me. Brake pad material is softer than the discs and will not build up on the surface of the disc. Not it the disc is flat anyway. The pads will always wipe off any material on the disc surface when the brakes are applied.

If you change the discs and pads you cure the judder. Simple.

Even if pistons stick, they stick evenly for each point of the disc surface. I say again, if the disc is flat to start with, then there can be no localised build-up of any sort.

I teach physics, if that helps.

mrcheerful wrote in news:8QHOf.30836$ snipped-for-privacy@text.news.blueyonder.co.uk:

Hi Mr Cheerful,

Thanks for the advice. I guess I just assumed there was a tool to measure brake piston sticking. If there isn't a tool, I can do what you say and test it by hand with the calipers disassembled.

Luckily these 1998 Toyota 4Runner front calipers are the easiest I've seen in my life to work on. We don't even need to remove a single bolt to get the brake pads out. All we do is remove a clip by hand, then pull two pins by hand, pop out the anti-vibration spring by hand, and then pull out the two brake pads and two anti-squeal shims per pad.

Looking in my Toyota Repair Manual, I can see that it only takes two bolts to remove the calipers from the car (in addition to the brake line).

Then it says to pry out the four "cylinder boot set rings and boots" per caliper, which at first I presumed was the same as the "piston seals" you speak of. But, reading on, Toyota says to blow out the pistons with compressed air (placing a wood plate in the space where the rotor was).

Only then, the repair manual says to pry up the four "piston seals" with a quarter inch flathead scewdriver. The Toyota Repair Manual exploded diagram shows four "set rings", "boots", "pistons", and "piston seals", in that order, outward from the rotor in both directions.

The problem then becomes, how does one test piston force after removing the piston seal when the pistons are already out of the calipers by this time.

Still, there's no harm in taking the calipers apart. The shop manual says to lubricate some areas with "lithium soap base glycol grease" and others with "disc brake grease". Aren't they the same thing?

So many questions ... so much to learn! Stu

This is not answering your question directly. However, from the discussion in the link, it is clear that whatever you pick, don't choose any grease that contains petroleum products.

"Why you should never use Copper grease anywhere near your brakes"

Or

mrcheerful . >

Many, many people spend a lot of money replacing discs when in fact it's worn suspension bushes that allow the suspension arms to wobble when braking that causes the shuddering. That is, unless the shudder is also coming through the brake pedal itself.

Personally I wouldn't bother checking the caliper pistons, as IME the chances of sticky pistons causing judder or shimmy is remote. The most likely cause is distortion or chilling of the disc. Chilling can cause hard spots on the swept surface of the disc. The uneven friction can cause judder of the brakes. The only cure in either case is to fit new discs, which is what I would do If I had the same problem. Mike.