Brake calipers have chromed pistons with a ground finish and zinc/cadmium plated bodies. There is minimal clearance between these parts and two seals; square O ring style for the brake fluid and a dust boot.

Normally the dust boots cracks up with age/temperature. Then the grease gets out and water gets in .. some time later the piston seizes in the body as the rust expands.

So first inspect your dust boots .. unless these are in good condition you are only buying 9 months of time by fettling the rest; source new ones if required. Then use a blunt set of mole grips or proper brake bars to seal off the rubber hoses .. be careful to only just nip the rubber hose supplying the caliper. Next make sure the bleed nipple moves as if it does not you will not be able to bleed the brakes for reassembly... a prerequisite to doing the job.

Now take caliper off .. you can either pump the piston out with the brake pedal (but be ready with the blunt mole grips when it pops out) .. or blow it out with compressed air into a rag ... air will push piston out with a fair pop.. if really seized you will have to pump it out with the brake pedal.

Inspect chromed piston .. should be unpitted but you can live with slight pits.. knock off all loose bits around pits so it is smooth - screwdriver's edge will do this if used as scraper not chisel.

Now look in body .. the square seal will still be in place .. ideally hook out with flattern hooked old bit of screwdriver .. seal picker.. funny sort of thing.. watch you do not damage it if you have no replacement. Now clear the crud out with small screwdriver .. and if required use 300 grit wet'n'dry or finer to clear to the top of the steel surface (doesn't matter about rust in the pits).. now reassemble with silicon grease (DO NOT USE ordinary grease as it makes the seals swell and you caliper will seize on again...) Make sure there is some grease between the square seal and dust boot as this is what prevents the corrosion .. result is caliper 75% as good as new .. should last 4-5 years if the chrome is OK on the piston.

.... now the quick and dirty way for an MOT .. lever piston back into body (take one pad out and lean on disc .. naughty again) .. I have used a 5 ton hydraulic puller to do this... then put old pads back in .. and pump piston back out.. repeat about 5-6 times and the piston will be free... nasty but what used to happens every spring to my old Lancia for MOT .. summer use only so just fine (and it has bigger brakes then the '5' with a 1300cc..) (11/99)

Seizure of the piston(s) is the primary cause of caliper failure, and is not at all uncommon. Malcolm gave a good summary of how to fix it - you don't need a new caliper - you do need a caliper rebuild kit (I wouldn't contemplate stripping the caliper and not using a kit, except in dire emergency), and you might need a new piston. You can sometimes resurrect a seized piston by polishing with the finest grade of abrasive paper - make sure you clean it completely afterwards.

I read that Blueprint do a rebuild kit. You have to find a Blueprint stockist - they aren't that common, although I know a few.

Anyone got the full list?

Also available by mail order from Finish Line Performance (miata.net marketplace)

Does anyone know if you can get new pistons, and where? (11/99)

My passenger's side caliper on my 1994 1.8 Roadster have in the past been very noisy. After greasing the sliding pins, replacing the pads, oiling the braking mechanism, replacing the piston seal, putting a small piece of tin into the pin receptacle (to prevent uneven pad wear), I'm still left with my same problem, which I think I can guess why.

I recently noticed that when replacing the pads on the opposite wheel, it had a seal around where the top pin slides in and out of the rear receptacle. The problematic wheel has this missing - is this causing the grease on the to pin to dry out/dirt enter, and thus inhibit the sliding mechanism? If I dismantle the caliper (which I have done on numerous occasions), and re-grease the whole thing, it is fine for 3-4 weeks - then the orchestra starts - giving the impression of drying out/getting dirty.

Phoning Mazda, the small rubber seal will cost £40 as part of a caliper overhaul kit. Ouch!

So:
(a) Anyone recommend a company that can refurbish calipers
(b) Anyone know where I can pick up a 1.8 caliper (cheaply)
(c) Anyone know if a 323 caliper will have the same rubber seal, and if so - where can I get one
(d) Will replacing this darn seal fix my problem? (6/00)

a) I don't think you need one, yet.

b) Ka Yu or Powerdrift et al.

c)I would guess they are, so try a Mazda dealer (but not the same one as before. He might remember you and suss out you are trying to save money.) Alternative there must be other makes of car that have the same little bit of rubber.

d)I would guess so. You could try moving the seal from the other calliper. If the problem moves then you have found the cause. (6/00)

Went over to Mazda to get the prices of the slider pins for the rear brake calipers. After much hunting around on their database, we found them: "pins, slider". Only, 2 were listed per side, with the same part number on each side. I thought they were different? They're not much; about £2.50 each or so, but can anyone confirm, before I order them.

Would ordering a seal kit be a good idea, or not necessary. I seem to remember when I looked at the seals, they looked ok, without any splits. (10/00)

I'm very confused, too.

I am absolutely, 100% certain that on each rear caliper there is one slider, which is a smooth cylinder with a thread on one end (on a reduced diameter) and either a hex socket or a hex shape at the other end (both varieties are around). See the illustration I referred you to before on Finish Line Performance. This goes into a blind hole in the caliper.

There is also a slider/bolt, which is a longish bolt with a smooth section to act as a slider and a threaded section (at the opposite end to the head) which screws into the mounting bracket. The latter is described as a lock bolt in the workshop manual.

The front caliper is different. This has two identical lock bolt/slider pins. they too have a smooth section and a threaded section, but the threaded section is below the head and the smooth slider section is at the end of the bolt. The difference is because the format of the mounting brackets for front and rear calipers are completely different.

I can only think that the dealer was looking at a pic of the front caliper. If the seals are intact (I assume you mean the rubber seals covering the slider bolts) they should be OK. I spray all rubber seals with rubber lubricant whenever I look at them - it keeps the rubber supple. Make sure that the seals are properly located in the grooves provided for them around the respective holes. (10/00)

Unseizing the calipers

Jack the car up and support it firmly - take the wheel off, release the handbrake.

See if you can move the handbrake pivot on the caliper. You may be able to identify if the cable is seized, or the pivot in the caliper is seized. It may help to remove the cable from the pivot. You may or may not be able to free either or both, but if one or other is seized you'll need to be planning on replacing the cable, and/or rebuilding or exchanging the caliper. It might be OK for a while, but it may happen again any time.

However, if both of those are free, proceed as follows:

Remove the plug from the back of the caliper (looks like a bolt), insert an allen key (4mm?) into the adjuster and see if you can retract the piston - don't use too much force or you'll damage the adjuster splines. Hopefully the piston will retract, but anyway, proceed to the next step. Remove the lower pivot bolt.

Try and rotate the caliper - a crowbar or tyre lever might be handy, but be careful.

You should eventually be able to get it to rotate, then when the caliper is clear of the disc and the pads, it might pull towards you off the top pivot. Use some force if necessary, again being careful.

If you're lucky, you'll get it off. In that case, check the condition of the rubber boots on the sliding pins. If they've got holes in, you'll need new ones, but it's not undriveable even if they are damaged. Grease the sliding pins - I use Finish Line Synthetic bicycle lubricant - black tube from Halfords Cycle dept.

Be very careful not to trap the boots when you reassemble. Adjust the handbrake with the allen key adjuster until the disc is locked, then back it off 1/3 - 1/2 a turn.

If the caliper won't come off the pivot, you have a problem. You need to disconnect the handbrake cable, the hydraulic hose (clamp it with a proper hose clamp) and remove the caliper complete with bracket. (2 bolts bracket to hub carrier)

Take it to a bench/vice. Hold the bracket in the vice and hit the caliper, preferably with something that won't damage it, such as a brass drift and hammer. Be very careful where you hit it, and only use just sufficient force. It will eventually come off. You'll need to clean up the pin and its socket.

Grease the sliding pins in the other side, and keep them greased regularly in future - at least once every 6 months.

If you feel you really need a new caliper, you could rebuild or get an exchange one - it looks as though an exchange one might be a better bet. (6/01)