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M/C Mechanics - HOW2:

Overhaul a Brake Calliper / Replace Calliper Seals'

This is NOT a definitive guide, nor is it a substitute for the PROPER Haynes / Clymar manual for the bike in question. Bike in question is a 1986 Honda CB125TD-C, in particular our 'Little Dream Number 3, "The Pup" ', but principle pretty much of a much for most bikes....

There are essentially two types of brake calliper, 'Fixed' and 'Floating'. Difference is in the mounting, and one has a calliper that is 'fixed' to the fork leg, with pistons in opposite sides pushing the pads against the disc in the middle. Other is 'Floating', which has pistons in only one side of the calliper, usually the outside, and its mounted on pins so it can 'float'. When the pistons push the pad against the disc on one side, they also push the calliper back, on the pins, pulling the pads against the disc the other.

The Callipers on the CB125TD are 'Floating' callipers, which means that we have to over-haul the 'float-mount' as well as the calliper. Fixed callipers don't have this so if that's what you have, you can ignore that bit. Other wise, float-mount designs and arrangements vary.

Now, starting at the beginning, there are a few reasons you might want to overhaul a calliper. You may want to do it as part of a full restoration or renovation, some 'precautionary' maintenance, or 'overhaul' or because you have a brake problem.

I'm going to start with the last instance, because an AWFUL lot of brake calliper overhauls start with a simple routine pad change, especially on rear brake callipers on bikes that have them, because they are often less used and get less frequent pad changes, so more prone to the phenomenon I'm about to explain.

In operation, the master cylinder pushes fluid down the brake pipe into the calliper, where it pushes the pistons out of the calliper housing, pushing the brake pads against the disc to create friction to slow the bike down. And disc brake callipers are naturally 'self-adjusting'.

{There Needs to be a Photo HERE of the brake calliper mounted on the bike, with the pistons, pads and disc showing.}

Every time you brake, you wear a little material off the brake disc and the brake pad, and with use BOTH get thinner. This means that the gaps between the pad and the disc get wider, so every time you brakes you would have to move the brake lever further each time to close up that gap, before the brakes started working, and the less brake lever travel you would have to apply braking force.

This is the same on a drum brake, you get the same wear between the shoes and the drum on those, but they don't 'self-adjust', there's normally a screw arrangement on the cable or rod that works them to take up the 'slack', that you should periodically adjust.

On a hydraulic disc brake, though its done automatically. When you press the brake lever, the master cylinder pushes fluid down the pipe to the cylinder and moves the pistons. When you let go of the brakes, though the ONLY thing to pull the brake pads away from the disc is the pressure 'release' between the pad and disc, and the spring effect of the seals between the calliper and the piston, that have distorted a little when the pistons were pushed out.

That is NOT a very big return force (unlike drum brakes that have big springs between the shoes to give a LOT of return force), so there is some 'lag' in the speed between the piston in the master-cylinder and the pistons in the calliper returning to their 'rest' position, and THAT creates a small vacuum in the hydraulic system, filled by the reservoir on the master cylinder.

So basically, every time you brake, you wear a bit of material away in the brakes, create a ting gap in the hydraulics, filled from the master cylinder, keeping the 'running clearances' pretty much constant, hence not needing adjustment.

Overall, then, to take up the wear, fluid moves from the master-cylinder reservoir, into the calliper, pushing the pistons further out of the calliper body, to take up the gap made by wear.

NOW, over time, the expose portion of the calliper piston, will get mucky and corrode. On older machines the pistons tend to be hard-chromed steel, on newer ones stainless steel, but either way the protruding end of the piston gets 'yucky', sat out in the elements.

THEN the brakes wear to the point the pads need changing, and to get the new, thicker pads into the calliper, you have to push the pistons back ALL THE WAY into the calliper body!

BEFORE you get to THIS point, its WELL worth having  GOOD look at those pistons, and cleaning them as best you can BEFORE you try pushing them back in the calliper body!

Very often, you'll find that the reason that pads have finally worn out is that if they are floating callipers, they have been 'sticking' on their mounts, and one pad is a lot more worn than the other. Or that one or other of the wiper-seals has given up, got gummed, or actually become dislodged, so that its not returning the piston, leading to a dragging pad and uneven wear. (applicable to floating and fixed callipers) OR the piston is cruddy and corroded and slicking in the bore, having the same effect!

But the MAIN thing is that you NEED to inspect the pistons before pushing them back in, because if they are crudded up, pitted or corroded on the exposed portion, when you push them back in, they will either stick, fail to seal or cut up the seals in the piston, leading to a brake problem or failure shortly after pad replacement. You HAVE been warned! So lets get on with it.

First of all, with the calliper STILL on the bike, and plumbed to the master cylinder, the brake pads need to be removed. This can be done with the caliper still attached to the fork leg, or if you preffer, removed, but still on the brake line. How you tackle it depends on your preference and the bike. Often easier to undo-stuff though when its bolted to something big and heavy, though.

{There Needs to be a Photo HERE of the brake calliper mounted on the bike, with the pads removed.}

Next, the pistons need to be 'pumped' as far out of the calliper body as you can get them, without them 'popping' out. Often easiest to do this with the calliper still attached to the bike, so that the disc stops them coming out too far, but a thin bit of aluminium or a bit cut from an old tin-can between piston and disc can help stop them gripping it so hard you cant get the calliper off later! If you remove the calliper, a tyre lever, bar, bit of wood or other packing can be used to limit how far out the pistons can come.

TAKE NOTE: if you have more than one piston, they are unlikely to come out at the same rate. One piston nearly ALWAYS lags behind another, even if one or other isn't sticking or completely seized! and if you keep pumping without something to limit the travel of the 'easy' piston, it will come all the way out, and leave you with no way of getting the others out, easily, and THAT will be the stubborn one!

On the CB125TD, its a twin-piston floating calliper, and both pistons are in the same side of the calliper body, which is a one piece casting with the opposite side relieved so that the pistons can be pushed all the way out of the housing and lifted through the other side when the brake pads are removed.

On SOME callipers, the calliper casting is in two pieces and the housing has to be 'split' to remove the pistons, or they have to be 'jiggled' out of the housing without splitting. this is more prevalent on 'opposed' piston capers, and on some designs the pistons are actually so long, that you can pump them out of the housing and they will meet in the middle before coming all the way out.

HERE! You REALLY need to check your work-shop manual, because in such cases its easy to think you HAVE t split the calliper, to get the pistons out, BUT many callipers are NOT diss-assemble-able, there are seals that are NOT service replaceable between the calliper halves and splitting the calliper CAN render it scrap. there is normally a procedure to hold the pistons in ONE side of the caliper, while the other sides pistons are removed, and THEN to work out the pistons from the other side. As said, THIS is not a definitive guide, nor substitute for the PROPPER work-shop manual for YOUR bike or brake. CHECK before you begin

However, moving on! Having removed the pads and protruded the pistons, the pistons can be inspected.

Shown here, removed entirely from the calliper, we have one of the original, chromed-steel pistons removed from the brake on Donna's 'Pup' when we over-hauled it, along side a nice new stainless steel replacement from Dave Silver Spares. I'll let you figure out which is which! If you cant fathom it, you shouldn't be attempting the job!

In this case the original piston, is pretty obviously 'scrap' there's hardly any chrome left on the thing and its as rough as a badgers behind, as my old granddad would have said!

for note, here is a selection of other calliper-pistons, in varying degrees of un-serviceability. NONE of these 'slugs' was put back into a calliper!

{There Needs to be a Photo/s HERE of useless brake pistons.}

IF in DOUBT - Swap it OUT!"

Its entirely subjective and you have to make a judgement call as to whether a piston is serviceable or not. And its a tough one. If a caliper is 'good' and the piston isn't sticking and the seal isn't leaking, then its often LESS risky NOT to disturb it, BUT if there IS evidence of corrosion on the piston, when you put it back in, it IS likely to cause problems, AND you have the risk that the new seals may NOT take, particularly on 'old' pistons. Personally, for the work involved, and given that the brakes are ONE area of any vehicle I DO NOT like there being any area of 'uncertainty', the cost of new pistons, though expensive, I reckon to be a small price for piece of mind, so reckon that If the seals need doing, chucking new pistons in at the same time, is worth it.

OK. Pads out, pistons protruded and inspected, time to get down to stripping the calliper for the new 'bits'. But first we have to take the calliper off the brake line. TAKE NOTE: If you have a duel disc brake system....... get ALL the callipers working off a master cylinder to THIS state BEFORE you remove an of them from the brake hose!

{There Needs to be a Photo/s HERE of hanging brake hose.}

Once removed from the brake-line, any hydraulic pressure you put into the system will take the line of least resistance. which means peeing out the end of the hose you have und-done. If you have another calliper to do on the other side the bike...... well you wont get the master cylinder to push the pistons out very far!

The brake line will also 'drip' brake fluid, and to protect paint-work and patio slabs or whatever, its worth doing something to 'catch' it all, like tape a plastic bag over the end or dangle the hose in an old coffee jar.

At some point you'll also want to purge the system of old brake fluid and refill and bleed. IF you are getting the calliper straight back on the bike, this MAY be worth doing now, to limit how much there is to drip, and avoid some-one squirting large quantities of fluid, inadvertently grabbing the brake lever! if you are in the middle of a resto though, and it may be some while before the calliper goes back on or the master cylinder gets any attention, better to leave the fluid in, as precaution against corrosion.

Next the Calliper needs to be removed from the float mount. These can be a bit tricky. They are either seized solid, or as in our case, it'll all just fall apart in your hands because of the wear, and or lack of rubber bits!

{There Needs to be a Photo HERE of the brake calliper being removed from the Float-Mount.}

Again, arrangements vary, check in your Work-Shop Manual for exact procedure! On the CB125TD though, there are two float pins, which is the normal number. Both screw through the calliper body into the calliper-carrier or float mount. One of them the calliper floats on directly, via a bush in the calliper body, the other indirectly, there being a sleeve on the pin, that is bolted to the carrier, the calliper body floating on that. No great problem until you come to put it all back together. Here and now you just undo the bolts and let the bits fall apart... or give them a bit of persuasion. But pay attention how they came apart, because you will need to remember what they looked like when you want to put them together again!

Then the Pistons can be fully removed from the Calliper Body.

Here Donna removed them the 'Brutal' way with a pair of mole-grips! IF you have protruded them properly while still on the brake line though, AND if they are in reasonable shape, they should just pull out by hand. If they are seized or sticky, they may take a bit more 'wiggling', but again, IF you protruded them as far as possible, they wont be a very long way into the calliper.

If you forgot to protrude them, or you couldn't do it under hydraulic pressure, then it can be a bit more tricky! Reason Donna is using the mole grips here is because the pistons WERE seized in this example, AND the master cylinder they were attached to had no fluid in it, and a reservoir cap with chewed screws so non could be put in.

Using mole grips, though WILL chew up the surfaces of the piston, so ONLY do this if the pistons are DEFINITELY scrap. there are special tools for removing pistons, like a pair of 'inside out' mole grips that grip the insides, but if the piston is so stuck you need them, I reckon the pistons are probably no good any-way, and if they don't need them, they will often come out with a blast of air from an air-line, on the hydraulic hose hole!

Once the old pistons are out, the seals can be removed.

This usually needs something 'pointy' to stab into the rubber CAREFULLY to prise the seal out of its rebate, WITHOUT scratching the aluminium of the casting.

There are two seals per piston, an upper one, the 'wiper' and the lower one the 'main' seal. The wiper usually comes out fairly easily, as its the closest to the top of teh hole, and you can get the best angle on it, and its often a narrower bit of rubber. Lower ones can be a bit more stubborn. Do NOT loose your patience, just work it carefully until its out.

{There Needs to be a Photo HERE of the brake calliper Bleed nippel.}

Dont forget to remove the Bleed Nipple! Often forgotten until the very end, and the calliper is back on the bike, new pistons in, and connected to the hose, and you have re-filled the master cylinder reservoir and are JUST about to bleed the system through! which is when you'll discover its seized, and Just to spite you the thing will shear in your lovingly reconditioned caliper!

THEN the calliper can be thoroughly cleaned, with hot soapy water. Here Donna uses an old drill bit to get all the corrosion, white fir and old crud out of the pad-pin holes.

This NEXT stage is entirely optional, but having removed and cleaned the calliper, during the Pup restoration, we prepared the surface with wire-brush, and sand-paper, masked and painted, so it looked good as well as worked good. This is NOT 100% necessary, but if you have gone to the effort and expense of reconditioning the calliper, and aren't in too big a hurry, it adds little time or expense and can make a big difference to the aesthetics!

Calliper Cleaned, brushed and sanded, re-washed to remove any cleaning debris, then oven dried, ready for masking.

Ordinary masking tape was used to mask ALL critical holes, starting here with the main brake-line port. A craft knife was used to cut the masking tape to the area to be masked, taking care not to damage critical surfaces of the aluminium casting.

The inside of the casting is not cosmetic. Once the pistons and pads are in place it will not really be seen, so this doesn't need to be painted, but most of it doesn't really matter if it is, either. The piston cylinders though we DO NOT want any paint into, so particular attention was paid to making sure these were well masked. As the holes are wider than a single piece of masking tape, three or four over-lapping widths were used, going along the length of the calliper, then more over-lapping runs of tape were used cross-wise to them to ensure that they sealed, and that the whole mask was firmly stuck to the calliper.

Less critical holes in the calliper casting were masked by rolling up a tube of paper, and letting that expand in the hole.

Calliper fully masked & ready for painting.

It was sprayed with High-Temperature paint, to withstand the heat generated in brakes, and cured in the oven.

Masking was then carefully removed, both so as not to damage the new paint finish, but also to avoid getting debris inside the calliper. It was then washed YET again, and again, oven dried, before rebuilding.

BACK to rebuilding the Calliper, then, the component parts laid out ready.

Main seals go in first, in the lower groove. They are the thicker ones, and are square in section.

Takes a bit of 'wiggling' but they do go in. Donna had an advantage here with little lady's fingers! Tip, wipe them with brake fluid first to lubricate them.

'Wiper' seals go in next, in the upper groove. These are the thinner rings and have a groove in the face. BOTH rings have to go in neatly without being twisted or kinked or anything. (Shown doing second cylinder's seals, for the more observant!)

THEN the piston can be manipulated into place, GENTLY.

Do NOT try and force it in to the seals, at this stage. It helps to just dip the end of the piston in fresh brake fluid to give it a little lubrication first, then it needs to be wiggled gently into the cylinder nice and square so you don't nip or twist the seals. You will feel it get tight as you start to get it into the seals. Turning it slightly as you go will help it go in.

As you get it past the second 'main' seal, you will feel it get a little easier as it is goes 'true', and THEN you can push it firmly all the way home.

Repeat for as many pistons as you have in your calliper, and that's the main bit of the job done!

Now the pads may be re-fitted.

First of all, 'Lightly' smear the back of each brake pad with Coppa-Grease, anti-squeal grease. Yes, I KNOW that I am 'buttering' the back of this pad rather err.... liberally! A light smear don't show up too well in 'photo's! Excess can easily be wiped off, and used on the other pad, or other parts of the brake. Just be careful not to get any grease on the friction material of the brake pad.

The Brake Pad retaining pins will also need a small smear of coppa-grease.

A small smear of grease on the pad-guide 'lands' of the calliper, and the anti-rattle clip, and the pads can be put into place.

If I don't have the calliper on the bike, and the brake disc to separate the pads and hold them near their seats, I find a bit of old card-board as a separator helpful.

Then the pad-pins can be pushed through to hold them in place, and locked off, to stop them coming out again. On this bike they are held in place by a plate with key-hole slots. Others have split pins, R-clips, threaded fasteners or other arrangements. As always CHECK THE BOOK! (note; Brake line port has been masked as the calliper was to be stored prior to re-fitment to bike)

Next the float mount can be re-assembled, using plenty of coppa-grease on the float-pins.

Getting the rubber boots over the end is a little more tricky...... its a LOT easier if you buy new ones. Old rubber tends to swell so wont stay on or go in to whatever its supposed to, or it goes hard and tears on you. For a few ectra pence when you are ordering the bits you need, these are well worth adding to the list!

Rubber caps / boots ' grommets, whichever you prefer to say, fitted, pins in, and tightened to specified torque, excess grease can be wiped off, and the brake calliper is ALMOST ready for re-fitting to the bike.

{There Needs to be a Photo HERE of the brake calliper Bleed nippel.}

Last thing that needs doing, as mentioned earlier when I said don't forget to remove it; bleed nipple needs putting in. again, worth adding a new one, AND the rubber cap to keep it clean to the parts list when you order the new bits. Do NOT over tighten it. It only needs fitting finger tight then a 1/4 turn to nip it up.

That done, calliper is ready to re-fit to the bike, by reverse of how you got it off, the system filled with new fluid and purged through and bled.


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