Rebuilding TwinPot Calipers

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[ARTICLE By Ofapars]

(Edited to be more of a Step by Step, and attempting to keep the same tone as the original article)

The usual issue is a sticking/binding caliper due to the sliding pins gumming up and preventing lateral movement, or corrosion under the piston seals, which prevent the pistons retracting easily. In both cases the pads will be dragging on the discs slightly leading to problems such as premature disc wear.

A CX/GL with twinpots that has stood unused for a `gap year` will nearly always have issues with the brakes. They like to be regularly used.

This is how i do mine,and the pics/text may give you an idea on how to tackle you own if you have sticky calipers.

    • Quick Service (Replacing Pads or a Yearly Refresh)

Piston Service:

  • Remove the caliper and then remove the pads (leave the caliper connected to the hose).
  • Place a suitable flat piece of material such as some 3-4 mm thick steel accross the face of the pistons and pump them out using the brake lever. One will probably move before the other hitting the steel, whereby the other will then start to come out. The pistons will not come out completely, when they both hit the steel plate stop. During this process keep an eye on the level in the resevoir as the fluid level will go down to fill the void left by the pistons in the caliper body. This will depend on the thickness of your brake pads and the level of fluid in the reservoir. If the master cylinder sucks in air you`ll have to bleed the system.
  • If the pistons have light corrosion but are servicable,they can be cleaned up using a gentle chrome pollish such as Brasso using a piece of rag wrapped around the piston,but be careful not to remove the gold-coloured plating though. If they are just dirty, clean using brake cleaner/de-greaser or just brake fluid. Note that there is no rubber boot enclosing and protecting the supposedly "high-precision" hydraulic pistons and they are completely exposed to road dirt/filth and the weather! OK, so they`re plated to resist corrosion because they`re steel, but not one of Honda`s best designs is it?

  • When the pistons have been cleaned, push them back into the caliper body using a g-clamp. You have to overcome the resistance of pushing brake fluid back into the resevoir through a tiny hole so it won`t be possible to do this with your hands. Do not use exessive force - the pistons have to be exactly square to the body or they may bind. Because the pistons are nearly all exposed at this time it is important that if you feel anything that suggests the piston jamming to re-align them to the caliper body and try again.
  • Apply the the g-clamp pressure to the opposite side of the piston to ease it into alignment. Protect the caliper paint with a piece of cardboard or rag. I don`t bother with that with my rat bikes.
  • Watch that the other piston does not move out too far when pushing the piston in due to the fluid pressure in the caliper. Keep an eye on the piston you are not working on in case it starts to move further out, another few mm and it`ll pop out. You could leave the steel plate across the pistons if you wish, but keep switching the clamp from one piston to the other and use only small increments with the clamp to ensure they go in square.
  • Watch the level in the brake resevoir - it will rise as the fluid in the caliper body is replaced, you may need to remove some if you added any during the previous stage.

Sliding Pin Service:

The long, bolt type one with the smooth shank and flat on one side can be left in place on the caliper - just clean off any old grease and also clean the hole it fits into on the caliper hanger. If you can remove/undo this bolt easily then do so, even just to clean the threads up and prevent it from seizing into place in the future which they are prone to do. You`ll have to try this first with the caliper still attatched to the hanger though, because it`ll most likely be very tight and it`s difficult to hold the caliper body firm enough while trying to undo the bolt. Sometimes the bolt head is damaged,too. If it won`t come out then no bother, it doesn`t have to be removed for a brake service/overhaul. The pin that gives most trouble is fitted between the two rubber boots between/below the pistons on the caliper body. Pull one boot out of the locating groove on one side of the pin and then push the pin out through the caliper body - if the brakes have been neglected it may be quite stiff - then remove both boots. If the calipers have been neglected for any period of time more than likely the rubber boots will be perished/holed/split or if the wrong type of grease has been used previously be swollen to a point where it will be near-impossible to replace them back in the locating groove of the caliper body. The type of material these boots are made from reacts with certain greases, they will then swell and deteriorate. If they swell enough they will "grip" the sliding pin, impede movement and you will have binding brakes and the associated problems. Clean the pin, and caliper body grooves where the rubber boot locates (see pic,i use an L-shaped piece of wire held in a pin-vice) - check for exessive corrosion on the pin and wear on the caliper body where the pin slides (high mileage ones go oval..), remove all traces of old greases to avoid contaminating the new rubber boots if they are to be fitted. It might also be a good idea when cleaning a bike to avoid splashing the brake calipers with degreaser type detergents in case they react with the sensitive rubber parts. If you are lucky the boots may be re-usable and will fit back into the caliper body.

If not,obtain new boots from Honda - there are two kinds, two of the type for the sliding pin,one for the other longer fixed pin per caliper. When assembling,use Castrol Red Rubber grease (or silicon grease,but RR is a lot cheaper, and you can buy a lifetimes supply for the price of a small tube of the silicon stuff!) - it is specifically for rubber brake components where swelling and hardening of components is to be avoided. Grease the hanger hole and boot (longer,fixed type), then install the boot in the caliper hanger. The other type is a little tricky - especially when everythings greased-up and slippery. There are a couple of ways of installing the boots`n`pins: One method is to locate a boot in one side of the caliper body (doesn`t matter which) - rotate it to ensure it is fully home in the (cleaned & greased) groove. Place the other boot on the sliding pin orientated in the way it will normally be when fitted, then slide it past the locating groove so that it is near to the opposite end of the pin but not sitting in the groove of that end - but close to it. Apply grease to the boots/caliper body,then install that boot in the other side of the caliper - the pin should be sticking out at this stage. When the boot is seated properly in the locating groove of the caliper (rotate again,this time holding the pin as well,try not to let the boot slip into the groove of the pin), you can then carefully push the pin in - it has to go over the resistance of the "seal" part of the boot,then overcome the second boot too - without displacing them from position. Easy does it, firm, smooth pressure with a bit of wiggling will do it... Another way is to locate one boot in the caliper as before but then install the pin only from the other side and push it through the fitted boot enough for it to stick out about half way. Apply grease to the relevant parts then install the second boot in the other side of the caliper, then push the pin into it`s working position through the second boot. It all sounds much more complicated than it really is in practice, both ways are equally easy/difficult/successful/efficient as the other, just see which one you prefer. I use method one mostly.. Check the boots are still properly located after fitting, and push the pin in and out/rotate the pin to ensure all is OK and to distribute the grease.

This pic shows one pin boot in position in the caliper body,the other boot on the pin ready to be inserted into the caliper.Use red rubber grease....!

After you have fitted the sliding pin re-fit the pads/retainer etc and you should be good-to-go. This mini-service is OK for calipers that have been working without problems or is useful if you just want to give the brakes a service to keep them fettled when they are working without any other problems, or while fitting new pads, but if you want to do a `full service`,recondition,or are still experiencing the calipers not releasing from the disc/binding, it can be extended to a full re-work to tackle the other reason they drag: corrosion under the piston seals. Pump the pistons out as previously described,then remove the hose from the caliper to enable working on the bench.Remove the bleed nipple and drain the brake fluid via the banjo bolt and bleed screw holes.If the bleed nipple is seized into the caliper body,be careful.Do not simply apply more spanner force,if the nipple snaps off you have a lot of extra work to do(or have a long delay while you source bleed nipple-equipped banjo bolts).Drain fluid and remove all other parts from caliper body and heat the body local to the nipple screw thread with a gas-torch with a small,localised flame.This will usually free up the nipple - i have saved several calipers this way. Pull out the pistons the remaining few mm with caliper piston pliers or regular pliers the jaws of which are covered with cardboard or other suitable material to prevent damage to the pistons. Use a small flat-blade screwdriver or piece of wire in a pin-vice to prise out the dust seal,and then the piston seal. Don`t nick the rubber.. You will probably find the seal-lands have white alluminium oxidation making the seals too tight a fit in the lands causing them to press harder onto the pistons,which resuslts in them not retracting easily when brake pressure (h`bar lever) in released. With a caliper empty of fluid and disconnected from the brake hose the (lubricated with brake fluid) pistons should be able to be pushed fully in/out of the caliper body by just using hand pressure if all is as it should be. If it takes a lot of effort it`s a sign that the seals are tight. Make up a wire tool (or pin-vice again) and scrape the deposits from the seal-lands,be careful not to remove any material from the caliper body.Work in good light and close-up so you can see all of the area to be cleaned, and make sure you get it ALL out, otherwise you`ll be doing it again in 12 months.. Clean/wash/squirt out all the dirt and make sure none is left inside the caliper body. Inspect the seals - the dust seals usually perish first, and replace any that are unservicable or look ragged, because if any water/salt gets in there you`ll be doing the whole job again soon. Scrape off any of that white deposit on the surface of them - i have found that in most cases the actual seals can be reused succesfully, but re-using dodgy dust seals will cause more problems later on.

This pic shows a caliper body in a seriously corroded state,it won`t be as bad as this in one that has just been removed from a bike but it shows where the oxidation forms under the seals;

During a brake overhaul recently i brought a brake caliper indoors to dry out. I just happenend to notice a piece of wire on the kitchen worktop which looked absolutely ideal for scraping out the dust/piston seal grooves. It was stainless steel and approx 1.5 x 3mm. About 10" long and sort of half-moon shape.... I cut a piece off and fitted it to a pin vice, nice`n`stiff, it ground down to a flat, chiselled point, and fitted the seal lands perfectly. Yep, it was a support bracket for one of Mrs Ofapars` undergarments... Oh dear, this is the second example of me using materials of a feminine origin on these web pages, but i can assure you it`s purely coincidence! I just use whatever is to hand, so to speak.. It did a fine job of clearing out all the oxidation;

The RatWing had been resting for well over a year, and was put away in the garage during the winter. Salt had been used on the roads. Of course the brakes seized up and needed a refresh. Here the new scaper is put into action and you can see the oxidation that has built up on the seals;

After the caliper body has been cleaned up,reassemble the caliper using brake fluid in the seal-lands and on the seals to lubricate and facillitate the replacement of the pistons,and dip the bottom of the piston into clean brake fluid to ease re-fitting. When everything is cleaned up properly it should be possible to ease the pistons back into the caliper body easily by just using hand pressure. If the pistons are gripped by a pair of pliers/caliper pliers you should be able to push/pull them in and out fairly easily, if they don`t it may mean that there`s still some crud in there which`ll have to come out. During the RatWing brake service i put a thin layer of water-repellant silicone water pump bearing packing grease in the seal lands to try to supress the formation of the oxidation. I`ll know in a couple of years whether it worked.. Using the `mini-service` guide above,reassemble the caliper.

The pic below shows the effect of using the incorrect lubricant (lithium grease,oil,coppaslip etc) can have on the rubber boots - these are the same part,but one has swollen so much that it would impede movement of the sliding pin when in position and would be impossible to re-fit in the caliper housing during a brake service.

This pic shows another example of the sensitivity of rubber brake parts;this master cylinder diaphragm was left in parts washer fluid overnight and the next day it was about one and a half times the size! I brought it indoors to dry out and after a few weeks it shrunk back down to its normal size;

This advise details the work to the calipers - there is another reason why the brakes may be binding,this is when the fluid return-orifice in the master cylinder body is blocked.There are two small holes in the bottom of the resevoir to allow fluid to flow to and from the piston/cylinder area.One is under a small metal cover which is the smaller of the two,the other is easily seen - make sure both are not blocked.The small one under the cover allows fluid to return to the resevoir and if it is blocked it allows residual pressure to remain in the brake system causing the caliper to bind on the disc(s). When the discs wear they form an `I` section, with the brake pads sitting in the thinner part of the disc, and a lip around the outer/inner edge. This mainly happens on Euro rear discs, the fronts don`t seem to wear as much. This makes it difficult to swing the caliper up and off the disc due to the brake pads hitting the thicker part of the disc. The pads/pistons need to be retracted. There`s no need to jam a screwdriver between the pads and disc to lever them back! That`s a bodge. Get a suitably-sized socket and place it on the rear face of the inner brake pad. You`ll probably have to turn the wheel to clear the spokes and provide access. Then get a G-clamp and place it between the socket and the front of the caliper body. Give it a few turns and the pads/pistons will retract into the body giving enough clearance to swing the caliper free of the disc;

If the bleed nipples are seized,i use a concentrated flame from a gas torch on the caliper body around the nipple,and gently turn it back and forth gently with a spanner to release it - be careful the thread doesn`t `pick up` on the soft alloy and damage the thread;

Here`s a few to service when i get the time....!;

Talking of brakes.......... The CX-periment nears completion and it`s time to sort out the front brake system.I need a master cylinder.A quick rummage around the `brake spares` drawer in one of the CX filing cabinets of assorted treasure provides a rather scabby looking plastic-resevoir-type master cylinder of uncertain model/marque or vintage.Its been left unloved for years with fluid oozing out,the paint has fallen off,theres oxidation all over it,it`s looking very sad and it`s seized up.Here we go again............... I can save it!

First thing to do would be to take the lever off,right? Woah,hang on,things arn`t as easy as that on this one - even the lever pivot bolt is absolutely stuck solid and needed the gentle persuasion and mechanical finnesse that only an impact driver and a 2lb hammer can provide,but several clouts later it yeilds to the prolonged process of percussive persuasion and comes away.I win. With that done lets try the notorious-for-seizing resevoir cap screws.It doesn`t look good.Someones already had a go at them and the rounded-off screw heads suggest some work with tools other than just a screwdriver may be required.

Sure enough,two of the screws came out OK but the others would not budge and the rusty heads just rounded off.I already had the drill prepared...... I bored into the heads a few mm with increasingly-sized drill bits until the heads popped off.I win again.You can`t let these mechanical objects get the better of you... Once all the fixings were off the first really exiting part of the overhaul was imminent - what would be revealed once the cap was removed? Actually it wasn`t as bad as i thought,there was still fluid in there which was still,err,fluid,it`s always been a gelatinous mass on all the other MC jobs i`ve done. With the cap off i could extract the remainder of the screws by gripping the protruding shanks with pliers.Look at those threads,no wonder it was seized. The resevoir cup was a really tight fit on the body - i couldn`t shift it with just my hands i had to use a lever to ease it off.

With the resevoir off i was not dissapointed in my jello fix as there was a nice fresh supply undeneath.A brake overhaul just isn`t the same without it..

Now to the next troublesome stage;removing the piston-retaining circlip. After teasing the rubber dust seal offand scraping away the fluffy crud i could just about make out the circlip hidden in the depths of the recess.I haven`t got a decent long-reach set of circlip pliers so what i tend to do is use the prong of just one side of the pliers to locate the hole at each end of the circlip,lever it out of the groove and then `walk` it up and out of position alternating between each end of the circlip so it doesn`t bind. When this circlip is removed the spring in the cylinder should push everything else out,but of course on this one it wont because it`s stuck. You could hold the MC vertically and apply some penetrating fluid and let it soak for a few days but previous experience has shown it didn`t make any difference so i`ll skip to the next stage - getting the recalcitrant piston gubbins out. The piston seizes in the `released` position so there is room for the piston to be pushed down the bore which should un-seize it. Tapping the end of the piston lightly with a small hammer in short-shock strokes is usually enough to free them up and they`ll retreat further down the bore before freeing up completely where the spring pressure will push them back up again.You have to be careful not to tap the piston too far down or you won`t be able to reach the dust boot groove on the end of the piston which is handy for putting a screwdriver in and gently levering the piston and seal assembly up and out once it`s loose enough.

Well,thats it dissasembled,does it look salvageable?,yeah no probs,and without spending any dosh,too..

Here it is after clean-up,everything looked fine,even the dust boot was intact - they usually perish first. What i do with the master cylinder body after scooping out most of the muck/jelly is to bring it indoors and put it on a radiator,heater or somewhere else warm.This`ll dry out the jelly and leaves just a white deposit which can then be scraped/rubbed and picked out and then cleaned using a piece of fine Scotchbrite pad. Then using some steel wire in a pin vice clean out the o-ring and circlip grooves. Master cylinders that have reached this condition invariably have the tiny fluid return port blocked or fused together,it`s only about 15 `thou or so in diameter and it`s easy to overlook if you don`t know it`s there,and will cause problems if left blocked.A piece of smaller steel wire again cleans it out.

With everything cleaned up and checked it was just a case of putting it back together with new res.cap screws. that i could start on the calipers. Here we go agian,how many times have i had to do this over the years.Bloody brakes.I`ve got four bikes and eleven calipers to look after.... I did the basic brake service first,and noticed the tell-tale coppery smugde of a previously applied lubricant on the slider pins resuslting in greasy grief - all the pin boots were swollen.......No bother,i had a spare `pattern` set here i bought via Ye Olde Interwebbe Spares Shoppe.The immaculate sliding pin was a surprise though,i`ve never seen one in so good a condion.There was just the one though,the others were the usual...The master cylinder performed without problems and bled easily. Bench set-up for a brake cleaning session:master cylinder mounted on a piece of tubing held in the vice and hooked up ready to pump the pistons out of a caliper

After doing a basic clean on both these front calipers i wasn`t sure if the pistons were completely free,they pumped out OK with hydraulic pressure and i was able to push them back with the g-clamp,but they still gripped the piece of steel i was using to prevent full travel when i released the brake lever,so i took the hose off and drained the fluid out to try something else; I fitted a Schrader valve to the bleed screw threaded hole and blocked the banjo bolt hole off with a suitably threaded bolt. Using a foot-pump i had to virtually jump up and down on the pedal to get the pistons to move and noticed the pressure gauge on the pump reading up to about 70/80psi.That didn`t seem right so i pumped them out as much as poss and removed them,cleaned out the seal lands and seals (medium oxidation found) and reassembled. With that done i hooked them up to the foot-pump again and found i was able to push the pistons out much more easily by just using hand pressure at about 25/30 psi.Good job i didn`t wait until fitted back onto the bike and found they were sticking on and have to strip them all down again..

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