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M/C Mechanics - HOW2:

Overhaul a Brake Master Cylinder / Replace Master Cylinder Seals

Brake Master CylinderThis 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 1984 Honda CB125TD-C, in particular our 'Little Dream Number 3, "The Pup" ', but principle pretty much of a much for most bikes....

Overhauling the Master-Cylinder on a motorbike isn't as common as overhauling callipers, the callipers sit down on the wheel in the muck, they get mucky, rusty and seize or stick, and there are a lot more moving parts. Master-Cylinders sit on the handle-bar, and as often or not, are completely forgotten, until new fluid needs to be added. However, they DO wear out, and sat there, often neglected, they can 'gum-up', and MANY curious brake faults are eventually traced to the Master-Cylinder. And they are REALLY easy to over-haul compared to a calliper!

Worth noting that this is the Master-Cylinder for a front disc-brake, and is of the more common type that has an integrally cast brake fluid reservoir. That's the square bit that sits on the top of the brake lever..... There are a couple of other arrangements, the other used on Front Brakes has a separate brake fluid reservoir, often bolted on a bracket on the lever assembly, most often round, with a screw top, and made of a yellow perspex, with a short low-pressure rubber hose between the reservoir and the cylinder itself. Other is the rear brake master cylinder, on bikes with disc rear brakes. These tend to be like older seperate reservoir front master-cylinders, only they dont have a handle-bar lever on the end of the cylinder, there's normally a rod poking out going to some kind of linkage aworked by the foot-brake pedal.  IF you can wrap your head round the external form of teh castings, though, the gubbins inside, and the procedure for oerhauling it all, is pretty much the same. So lets get on with it.

FIRST of all. DO NOT take a Master Cylinder apart unless you have new seals to go in it! (Or you don't want / need to put it back together again!) The seals are rubber and brake fluid contains special solvents called 'seal-swellers' that soak into the rubber and make it softer, and bigger... swell. This helps improve the seal it gives. BUT take the seal out, and you relieve the pressure that its swelling has created, and two things happen. First, even if the seal isn't perished or worn and 'LOOKS' like it's 'OK', you'll struggle to fit it back in its hole.... its now grown and will be bigger than when new! Second, IF you manage to get it back in, it will be 'saturated' with old seal sweller, and wont expand again, so it wont swell up to give the same sealing pressure, and it wont seal very well. At BEST old seals might 'just' work. If they do, they will tend to be spongy and not work very well, or for very long, in the worst case, they will jam stuff up or completely not do the job. so we just DON'T GO THERE.

Photobucket ALWAYS buy NEW SEALS when doing anything on brakes, OK? and while your at it, on the Master-Cylinder, as its such a simple assembly, and by comparison NOT very expensive to do s, you might as well get a complete over-haul kit, which will comprise not just of the seals you need, normally two or three, but also new return spring, probably the piston, and usefully new circlips and dust boots and stuff' to make it a proper job.

Leaping ahead a bit, but pic shows our candidate master-cylinder cleaned and stripped with the working gubbins from inside it laid out on the bench, with the new replacement parts of the over-haul kit laid out beneath.

Talking you through it; black casting is the cylinder body, beneath from left to right is the return spring & bottom seal, Piston, (with piston seal still fitted on the old part above), the retension circlip and the dist boot..... THAT is pretty much ALL there is to it!

Job, is to get all them bits out the cylinder, and fit the new ones in....... so lets get on with it!

{There Needs to be a Photo HERE of the Master Cylinder on-bike, hose removed Caliper end draining into Jar.}

First job is to get the Master-Cylinder off the bike. This will probably entail removing the brake hose, and draining the brake fluid. Note that as soon as you have removed the hose, from the master-cylinder, when you pull the brake lever in it WILL squirt brake fluid every-where and the stuff EATS paint, like teen-age kids devour everything in the fridge! Best to remove the hose at the caliper end, and pump ALL the fluid out into a jam-jar or similar, THEN remove at the master-cylinder, and as there WILL be residues in there, plug the port if you can, or be especially careful not to get brake fluid everywhere.

Next the master Cylinder needs cleaning, & in the case of the integral reservoir type, the cover removing. In THIS case one of the screws was seized in the casting, and as normal, the head rounded out by some-one trying to remove it! (They failed, & we thing threw the whole brake system away & replaced it, rather than perceiver, as this whole assembly was one acquired as 'spares')

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In this case, the screw needed drilling out, and IF you are careful, as I was here, you can drill off JUST the head of the screw so you can lift the lid off over the remaining shank of screw, and then get a pair of mole-grips onto it to grip and turn. If you aren't careful, you can chew up the reservoir lid, or the hole, or the boss that the screw goes into and scrap the whole assembly.... see other how to's for the best way of removing stubborn screws and fastenings!


Pretty good bet that your cylinder will have accrued its fair share of crud and corrosion over the years, so next thing to do is to clean it. Hot soapy water works well, and something 'pointy' to get in all the orifices & crevices, especially around the end of the cylinder. If the old dust-boot is still present, you may want to pull it off to get in there a bit better.


IMPORTANT!!!!! Before poling around too much to remove the retaining circlip....... LOCK THE PISTON!

The piston is spring loaded, that will put load on the circlip making it harder to remove, and when you do get it out, everything is likely to come shooting out! There will be a port in the side of the cylinder inside the reservoir, or where the reservoir connector pipe attaches if its an external reservoir cylinder. Find something hard and the right diameter to go through that hole (The back end of a drill bit normally works well) press the piston in against the spring, and push in your pointy thing to hold it down while you tackle the circlip.

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In THEORY you should be able to remove the circlip with an appropriate pair of circlip pliers..... In this case though (& I suspect if haven't actually proved, many, many others) your circlip pliers wont go into the recess to get at the thing AND get some purchase on it! I have MANY MANY pairs of circlip pliers, some even modified by grinding the tips to get into awkward places, & this one defeated them all.... So patent pokie tool was used. Note: some initiative & LOTS of patience MAY be required!


Circlip removed, whatever you locked the piston with may be removed & the piston will come shooting out, as shown in this very well captured action shot! We picked the pieces up off the floor later for photo's...... err... WORTH keeping your hand or a jar or pointing the cylinder at a tub or some-sort to catch the bits.... to be honest!

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All 'gubbins' removed, the bits that came out can be cleaned for inspection, though probably NOT re-Use, while the master-Cylinder itself needs cleaning METICULOUSLY. again, hot soapy water works wonders. In this case, the old brake fluid had decayed to jelly in some places, or just left a lacquer like coating in others, and a soft nylon (tooth) brush, patience and persistent scrubbing was the best way to remove it all. THAT was the main problem with this assembly. In others the main problem will be wear or scoring of the actual cylinder, which needs to be inspected carefully, to determine if its worth reconditioning.


Shown you this pic before, & explained its contents, but once all gubbins removed and master cylinder cleaned, and dried (I put castings like this in the oven on a low heat for about 20min to get them properly dry with no residues), the assembly can be prepared and rebuilt.

Master Cylinder, painting

In THIS case, (& Many I've Done) the assembly was painted before being re-built. THIS IS OPTIONAL.

As mentioned, brake fluid & paint aren't the best friends, and painting a master-cylinder can be a very futile as well as messy job. As soon as you put brake fluid into the finished assembly, or even before, small drips or spills or residues on fingers or gloves will instantly attack and crinkle your lovely paint! The castings ORIGINALLY aren't always painted, they have a surface treatment that makes them black or silvery coloured and actually stains the metal, and that isn't attacked by brake fluid. Some-times special solvent resistant coatings are used.

IF you choose to paint brake components, and particularly quite obvious 'cosmetic' ones like the handle-bar mounted master-cylinder, FIRST of all you need to be ESPECIALLY careful to mask the casting very carefully to avoid getting any paint over-spray inside the workings. If you DO, as soon as it is touched by brake fluid, it will get attacked, will peel off, contaminate the fluid and risk gumming up the workings or blocking fine ports or oil-ways. NEXT, you need to be very careful about choosing your paint, and picking one that is solvent resistant, or better still, brake fluid resistant. Brake Calliper paint isn't always fluid resistant, BTW! Here we used conventional panel paint. all the handle-bar fittings were colour matched to the bodywork, which is 'not-quite-black' dark metallic purple. This would NOT be brake fluid resistant, however, we used solvent resistant lacquer on the top coat. This proved NOT to be 100% resistant to brake fluid, however, with two coats of solvent resist over five coats of normal lacquer, when it was touched by brake fluid, it did NOT manage to attack the paint beneath the lacquer, and merely stained the top lacquer coat, WHERE I was cautious to not allow the brake fluid to linger, and resisted temptation to 'wipe' off, and so spread contamination, but washed with lots of water, immediately. Stain subsequently polished out after the cylinder had dried, and I applied a 'touch' up of solvent resist lacquer after, to repair the protective coating.

Motorcycle,Mechanics,Honda,CB125TD,Switch,Tapping Motorcycle,Mechanics,Honda,CB125TD,Switch,Tapping

After Painting, the cylinder was 'fettled', all tapped holes re-tapped to ensure that the threads were clean, and the mating surface of the reservoir dressed on fine wetted wet & dry sand-paper to get a good seal. This is good-practice, even if the cylinder has not been painted or coated.

Motorcycle,Mechanics,Honda,CB125TD,Clutch,Master Cylinder Motorcycle,Mechanics,Honda,CB125TD,Clutch,Master Cylinder

Soaking new seals Fitting return spring & seal Fitting Master Cylinder, piston seal

With the cylinder casting fettled, the internal 'gubbins' can be fitted. First thing, is to soak the new seals in a little new brake fluid. DO NOT let them stay in the brake fluid too long, this is JUST to lubricate them, and get a little seal sweller in to soften them. IF they are left to soak in the fluid too-long they will swell too much, and become useless like old seals.

The return spring & end seal can then be fitted into the cylinder, before fitting the piston seal to the piston. THIS is the tricky bit.

Fitting Master Cylinder, piston seal Fitting Master Cylinder, piston seal Fitting Master Cylinder, piston seal

First you need to make sure that the piston seal is facing the right direction on the piston. Refer to the MANUAL or the instructions that come with the over-haul kit to ensure you have it the right way round.... NOT the old seal. It may NOT have been fitted correctly to begin with, and wear and sweller can distort it making it appear contrary to how it should be.

The seal will be tight on the piston shaft, and you'll probably have to manipulate it over or past some steps in the diameter, that can be quite large. THIS is why you soaked the seal, so that it is soft enough and slippery enough to be manipulated into place without tearing or snapping or snagging.

As shown I carefully used an old small screwdriver to lever the seal over steps and into its rebate. CARE must be taken NOT to stab or nick the new seal, do NOT push with the point of a pointy thing or screw-driver!

Fitting Piston to Master Cylinder

Once the seal is fitted to the piston, the piston can be fitted into the cylinder, and locked in place with the retaining circlip. As before, it can be helpful to lock the piston down to hold it in place and relieve pressure on the circlip, by pushing it all the way down and using something through the reservoir port to lock it in place, like the back of a drill bit. again, as for removal, circlip pliers or patent pointy-thing, initiative and patience may be used to locate the circlip.

ENSURE that the circlip IS properly seated and will NOT let the piston come out of the cylinder before deeming the job done!

Fitting dust seal

The dust boot can then be fitted over the end of the piston, a little grease can help it seal and deter corrosion. Be careful if using something pointy to seat it in its rebate, not to tear the thin rubber.

Master Cylinder Assembled

The cylinder is now pretty much reconditioned, and all that needs doing is making sure everything else is put back in in place, in this case the clip over the reservoir port, inside the reservoir, the reservoir seal and lid.


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