Re: Can anyone check whether brake pistons move freely?

If you have more than 75k miles on them just replace them. You could just pull them off and have someone hit the brakes and watch them close.

You'll see that though. & it's a race car issue, not a road car one. Most raod car judder seems to be soft suspension bushes exagerating small amounts of warp.

the easiest way for you to satisfy yourself as to the ease of piston movement is to strip the caliper completely, with no piston seals at all the cleaned pistons should be able to fall freely in and out of the caliper.

the initial problem is usually a heavily worn disc or a disc that has excess runout, this leads to disc thickness variations which is what most people call warped discs.

replace the discs, check that they do not have excess runout, and that the pistons move freely and that the pads are not tight in their locations, all will now be fine.

mrcheerful

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

Just to break in, in case you're just about to do the job ;)

Mr Cheerful's method doesn't involve any dissasembly of the caliper for the test he describes.

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"

Oops....

I did my SD1 Rover front calipers recently and these have 4 pistons each. After removing from the car, use a bleed nipple(s) from the other side to seal the pipe(s) inlet. Get a rubber tyre valve - the type used in tubeless tyre wheels, remove the valve core, and fit it to a tyre compressor or foot pump. Secure all the pistons using G Clamps - etc. Push the tyre valve onto a bleed nipple which is open and apply air pressure and release a G Clamp gradually till one pops out. Be careful as fluid will go everywhere and there is considerable force around. Once you've discovered how far they have to go to come out you can adjust the other G Clamps to stop them only just in time. You then put the one that's come out back in just enough to make a seal.

Examine the pistons and bores for damage or corrosion after a thorough clean. Damaged piston replacements can usually be bought separately.

What you want is red rubber grease. Most decent accessory shops should have it.

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.

Care to explain what "chilling" is?

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Stuart A. Bronstein" saying something like:

Biggest load of tripe I've ever read.

Place not thy faith in internet gurus, my son. They knoweth not of which they speak.

If you follow this advice, you might as well run a hone down the bores and put a new seal kit in the calipers, assuming you can find same. That will be the same as buying a "rebuilt" caliper from your FLAPS and will be much less expensive

Of course, you should also clean them well and spray them with Ford Red paint while you have the castings bare :)

After you are satisfied that everything is good and clean, but before reassembly, you should wash all parts in ALCOHOL (nothing petroleum based) and lube the seals with either brake fluid (will destroy your nice fresh red paint, though) or silicone grease.

good luck,

nate