(click on pictures and links to open a larger image in a new window)
** Wear Safety Glasses **
Before you go pulling calipers apart, try cleaning and lubing the pistons. Using a thick washer, like in picture (4), extend the pistons fully and clean the piston surfaces with a cotton bud soaked in DOT fluid, a silicone spray lube. or plumber’s silicone (something like Hunters) The tolerance between the pistons and their bores is quite fine so it’s not unusual to get some dirt build-up, especially if you ride in particularly dusty conditions. Extend the pistons, clean with cotton bud; drip/spray silicone over pistons; push pistons back into caliper using a plastic tyre lever. Repeat the process two or three times, then fully dry the caliper interior, refit pads and try the brake’s operation again. If a seal has failed, it will usually be evident by the refusal by the piston to return after the brake lever is released.
The following guide is for overhauling Hope’s Mono calipers (the corresponding guide for the Mini/Mono lever can be found by following this link.). Mono simply refers to the caliper being cut from a single billet of aluminium, rather than the two-piece design used by the original Minis or closed caliper brakes, so all Mono (Mini, Trials, M4, V2 and M6) are covered here. If you can do a seal change on a Mono Mini, then you can do a seal change on an M6. Special tools are required for this procedure as the piston bore cap has a splined interface for removal/fitting. Unless you’re fortunate enough to be able to machine or manufacture a suitable tool yourself, I would strongly advise against trying to remove a bore cap with anything other than the appropriate Hope tool. There are five different tools, each with specific capability…
• Mono Mini
• Mono M4 large cap
• Mono M4 small cap/M6 large cap
• Mono M6 small cap
• Mono Trial/V2 cap
The other tools required for the job would be an 8mm spanner for the banjo bolt at the caliper and for the bleed nipple; a 2.5mm Allen key (Mini) or T10 spline tool (Moto) to remove the master cylinder (MC) cap bolts; a very small point, like the jeweller’s screwdriver shown below, to remove and fit the seals. This can be quite a messy procedure, so keep a roll of tissue paper handy, as well as the mandatory bottle of Isopropyl Alcohol (IsA). Did I mention safety glasses?
Seal kits should contain one flat-edged seal per piston; an ‘o’ ring for each bore cap; and a cap for the bleed nipple, as seen in this seal kit for the M6 caliper being overhauled here.
The caliper should be removed from the bike, although the lever can remain on the bars and, for the moment, attached to the caliper. The pad retaining clip can be removed by gently pushing the looped section up and over the end of the pin with a small point (1), although a fingernail is good enough if you have the length. Make sure to keep another finger ready to stop the clip flying off when it pops over the end of the pin. With the exception of the V2 (and possibly the Trials), where the pad pin unscrews (3mm Allen), the pad retaining pin can now simply be pushed out of the caliper (2). Remove the pads/spring (3) and, if you’re going to reuse them, put them somewhere where they won’t get splashed with DOT fluid.
Because the Mono calipers only allow direct access to the outboard piston(s), we can make our job a lot easier by extending the inboard piston(s) out of the caliper before we go any further. I have a thick washer (4) that fits inside the rotor aperture on the Mono calipers and which I can use to extend the pistons without them coming free of the caliper (5). If you have a spare rotor lying around, this will do perfectly, but take care not to scratch the pistons or caliper body. Outboard pistons can be pushed back into the caliper using a plastic tyre lever (6) before depressing the lever again to extend the inboard piston(s) (7). This procedure ranges from ‘simple’ on the Mini to ‘mildly frustrating’ on the M6, so perseverance and patience are necessary on the multi-piston models. Remember that as soon as one piston is free of the caliper hydraulic pressure will be gone, so keep them moving evenly until you feel that each can be moved the last step past the seal by pulling with your fingers. This step can be made easier with a helping hand to pull the lever while you control the pistons.
(The small pistons of the M6 can be awkward to get at if your fingers are over a certain size, so you could try some long-nose pliers with a small piece of inner tube to protect the piston. I can’t stress enough the amount of care required if you resort to this method. A scratched piston will almost certainly require replacing as it will damage its seal and possibly even the caliper itself. Go slow, manipulate rather than pull.)
With the inboard piston(s) removed (pic), the hose can be removed from the caliper, so get a piece of tissue handy and remove the banjo bolt with an 8mm spanner (8). If you can keep the caliper above the height of the lever while you do this, spillage will be kept to a minimum. Keep the caliper tilted back slightly to stop fluid leaking. When the bolt is out, wrap the tissue around the banjo (I leave the bolt and copper washers attached) and secure it with some tape or a cable tie (turning it inside out so that it can’t lock makes it reusable). I then pop the wrapped end behind the hose at lever (9), securing it with another cable tie if necessary.
The outboard pistons are considerably easier to remove. The splined bore caps, which also comprise of a rubber ‘o’ ring seal, can be in quite tightly, so, if you feel as though you’re having a struggle, just bolt the caliper back onto the bike (careful not to get DOT on paint or rotors) which will allow you to concentrate on holding the tool rather than gripping the caliper. If you have a tool with a rounded shaft, like a screwdriver, then use this to act as the handle for the bore cap tool (10). Otherwise, a 5mm Allen key does the job. Make sure that you have the caliper gripped firmly in one hand, whilst keeping a firm, even grip on the bore cap tool and its handle, pushing the cap and tool together while turning. The caps and cap tools aren’t as tight a fit as I think they could be, so guard against the tool slipping out of the cap. When the initial torque has been released, the handle can be dispensed with and the cap removed by turning the tool by hand (11). As appropriate, drain each piston bore as you open it (12).
With the cap(s) out, the piston(s) can be removed. To prevent any damage to the piston(s) or caliper, cut a piece of card the same width as a brake pad and slide it into the caliper to ‘catch’ the piston(s)(13). Put a suitably sized tool inside of a cloth and use it to gently push the piston(s) out into the pad/rotor aperture of the caliper (14).
To remove the seals, carefully insert a point or small screwdriver behind the seal and prise it forward (15). Each seal should crimp forward and allow you to move your tool around the seal aperture until the seal lifts out (16). Sod’s Law says that inboard seals will eject themselves into the bore rather than out to freedom, forcing you to root around to get them out…
With all the seals out of the caliper, all that is left is to clean the body down thoroughly. Try to resist rinsing with water or IsA as it’ll only end up trapped in the fluid routes of the caliper and make bleeding a nightmare or future performance suffer. A scrunched tissue can be threaded down the piston bore (17) before a cotton bud with a little IsA on it is used to get into the nooks and crannies of the seal apertures (18). Remember to dry all surfaces, too. The bore caps should be checked for ‘loose’ threads. I’d say about 70% of all caps that I’ve removed have had shavings of aluminium on them from the caps themselves or the thread of the caliper. Ensure that any ‘swarf’ is removed from all threads.
Fitting the new seals should be pretty straightforward. Again, things get a little more finicky the more pistons you have, but patience is an excellent tool. This part is basically guideline; the seals can bend and move into various unhelpful shapes, so although the process of lubing and fitting is typical, you’re going to have to develop your own method for actually getting the seals seated in the caliper. You should put a drop of DOT fluid or silicone grease into the seal aperture (19) before rotating the caliper to spread the fluid right around the internal surface. This lubricates the seal evenly to prevent it from sticking to the caliper and affecting the movement of the piston. Also, lubricating the aperture rather than the seal makes it much easier to manipulate the seal into place. Each seal can be nipped gently to elongate it enough so that it can be easily placed in the seal aperture (20). A combination of gently finger pressure and a plastic point is best suited to poking the seals into place. It is a low force task, and so a needle or small screwdriver is perfectly alright for ‘pressing’ the seal into place so long as you tread carefully. Once the piston seals are fitted, the pistons themselves can be reintroduced to the caliper. The phenolic pistons most commonly found in the Mono calipers are marked with a letter/number on the outside face; the opposite (internal) face has a bevelled indent (21). Use a cotton bud to get a good coating of DOT or silicone grease to each seal before inserting the piston (22). Take care to ensure that the piston is lined up perfectly with the bore and insert outboard pistons from inside the rotor/pad aperture, not down through the bore. The bevelled edge of the rear face of the pistons will pass over the square edge of the seals much more easily. You may find it helpful to drip a little DOT fluid around the piston before pushing it into the caliper. This can be done from two sides on the outboard piston(s) and will usually make for a smoother insertion. It can take quite a bit of force to fully depress a piston past a newly fitted seal, but always double check that the piston is straight with the bore before increasing your force.
Once the pistons are all replaced, the bore caps (with new seals) can now be refitted. Remember to wipe a little DOT around each seal before you thread the bore cap in. Take time to ensure that the threads aren’t crossed; you should be able to thread the cap in easily using the bore cap tool by hand, no handle. If it feels too tough, back off and start again. The caps can be torqued fully once they are fully seated by introducing a handle to the tool. Your caliper is now ready to be reconnected to the hose and the system bled and put back into action. It’s usually worth pulling the pads after your first ride as there will probably be a small residue of DOT fluid left over from the seal insertion process. Give the back of the pads a wipe and also the inside of the caliper around the pistons.
Follow this link for instructions on bleeding your brake.
This guide and the photographs contain within it are my property and as such are covered by copyright. Please feel free to provide links to this guide, but do not copy or reproduce any part of it without my permission.