Mountain Bike Maintenance

Fix your own bike…

Hope Mini/M4 (two-piece) Caliper Overhaul

with 5 comments

(click on pictures and links to open a larger image in a new window)

** Wear Safety Glasses **

This is a guide for a complete overhaul of the original two-piece Hope Mini/M4 caliper. The corresponding guide for the Mono (one-piece) caliper can be found by following this link. Manuals and schematic diagrams for most Hope products can be downloaded from direct from the Hope website. Bleed instructions can be found by following this link. The overhaul guide for the Mini lever can be found by following this link.

These are the torque settings that Hope advises for the Mini caliper;
Hose Connector 8Nm
M6 Bolts 8Nm
M5 Bolts 4Nm

DOT fluid is toxic and should be properly disposed of. Most of us will only have tiny amounts to deal with, so you could just leave it in a container and drop it into an auto shop when the container is full. Shops have to pay to have their waste disposed of, although I’m yet to be charged for them taking my occasional half litre. Do NOT pour waste DOT into a public drain (sink/toilet).

Through experience I’ve found that reusing rubber seals can sometimes be problematic, so, to be on the safe side, it’s worth having a set of new seals ready. Seal kits are inexpensive and available from suppliers of Hope brakes. This guide assumes that you will be using a full new seal kit.

Hope brakes are designed for use with DOT 4 or DOT 5.1 brake fluid. Use of mineral oil or DOT 5 fluid will damage the rubber seals used throughout the system. Hydraulic fluid is an irritant, so safety glasses should be considered a must when working with it. If you do get any in your eyes, rinse immediately with water and seek medical advice if necessary. Hydraulic fluid will also damage or remove paint if left in contact for long enough, although it can be easily washed away with soapy water or wiped with a cloth and a little Isopropyl Alcohol (IsA).

I’ll start with the caliper seal change. Although it’s not 100% necessary, at least for the lever, it’ll make your job much easier if you disconnect the lever and caliper from the brake hose. I drained the system when I did this procedure, but you should be able to retain the fluid from the hose if you wish. First of all, remove the wheel and brake pads. If you have a work stand; no problem, otherwise you’ll need to ‘jack the bike up. As you can see in the pictures, I just rest the chain stays on a small toolbox. The pads can be removed by unscrewing the pad retaining bolt that runs through the caliper. This may have a metal ‘R’ clip on the other end; just slide it off. Slide the pads and spring out of the caliper and put them somewhere where they won’t get contaminated.

It’s worth having a small container handy to drop all the components into as you remove them. I usually drip a little IsA onto each part if necessary as I remove it, with a view to cleaning/drying thoroughly before refitting.

You can make piston removal a little easier by inserting a rotor (or similar) into the rotor aperture of the caliper and pulling the lever until the pistons have extended.

To drain the system, first attach a length of pipe to the caliper bleed nipple (1). I send mine into a hole pierced in the top of a jam jar, but you can just drop the end into a pot/jar (remember that it needs to be lower than the caliper). At the lever, remove the top-cap and diaphragm from the reservoir (2/3). Using an 8mm spanner, loosen the bleed nipple a half turn (4) and allow the fluid to drain before removing the pipe.

Place a cloth/tissue to catch any spills and use an 8mm spanner/socket to remove the hose bolt assembly (5). Holding on to the hose to remove any tension will make it easier to remove this bolt, and also reduces the likelihood of damaging the threads. Wrap the hose end in tissue and tie it up (6). It is not necessary to remove the connector unless you’re shortening/replacing the brake hose. I’ve only done it here for the purposes of illustration; picture (7) shows the assembly: two copper washers; hose connector; and bolt and the brass olive. Leave the bleed nipple in place as you’ll need it for getting the pistons out. Remove the caliper from the frame, making sure you collect any washers/shims that are used for centring the caliper/rotor, taking note of which/how many came from which bolt. Remove the two bolts (4mm allen) that hold the caliper halves together (8). It is possible that these bolts will be very tight, so you may need to hold the caliper with a cloth, or even bolt it back onto the frame to allow you the necessary leverage.

Split the caliper into its two halves (9) and remove the small rubber ‘o’ ring from its recess. NB. The piston seals in the Mini caliper are Hope only. The small ‘o’ ring, however, can be found in the plumbing section in most large hardware stores. To remove the pistons from the caliper I use an old bleed nipple, a length of 5mm pipe and my tyre pump (10). A track pump works best for this. The pipe makes the job a little easier and also stops the pump from being contaminated with DOT fluid, as it will cause the rubber to perish. Note the plumber’s PTFE tape on the nipple. Screw the bleed nipple into the caliper half, attach the pipe/pump and back the nipple off a half turn. Place a cloth over the piston (important!!) and your thumb over the fluid hole (where the ‘o’ ring was) and compress the pump. You may have to pump it a few times, but the piston should eventually pop (literally!) out of the caliper body. The cloth should catch the piston and any fluid that is ejected from the caliper. Transfer the pipe assembly to the other caliper half and repeat the process.
You’ll now have two halves and two pistons (make a note of which piston came out of which half). Depending on how long the brake has been in use the pistons may have a rim of dirt (11). This should only be removed with a cloth/tissue. Resist the urge to use an abrasive cloth or glass-paper as the piston is a precisely machined component. Use IsA if the dirt is particularly stubborn, or soak in WD40 for a few minutes before removing the dirt with a cloth.

The rubber seals in the caliper (14) can be carefully removed with a small point or jewellers screwdriver (15). Do take care not to scratch the caliper surface/piston bore. (16/17) Clean both halves of the caliper down using IsA and prepare the new seals. Remember to make sure that the caliper is entirely free from IsA before reassembly.

Dip a cotton bud into some DOT fluid and run it around the recess’ for the seals. Also apply a thin layer onto the whole of each seal. Carefully push the each of the seals into their recess’, making sure they are properly seated. Apply a thin layer of DOT to each piston and push it slowly into its piston bore (18). Rub a thin layer of DOT to the ‘o’ ring and its recess’ and drop it in. Note that one half of the caliper has a deeper recess for the ‘o’ ring. Thouroughly clean the facing surfaces of each caliper with IsA (19), ensuring there is absolutely no dirt or grit which may prevent the two halves mating up cleanly. Push both pistons fully into the bores and bolt the two halves back together. Don’t screw one bolt in then switch to the other; thread one in a little and then do the same with the other so that compression is similar on each. When the caliper halves are fully torqued together the pistons will need to be pushed fully into their bores in preparation for bleeding the brake. I use a block of pine wood that I filed to the correct thickness (20). The distance between the two halves of the Mini caliper is 11.5mm.

Instructions for bleeding the system can be found by following this link.

Written by SteveUK MTB

April 6, 2008 at 2:59 pm

5 Responses

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  1. Hi Steve,
    Have you ever experienced a problem with the brake fluid escaping from between the caliper halves following the rebuild? This is happening to me – it’s as if the o-ring isn’t keeping the fluid in. I’ve tried to keep it all clean as I rebuild, so I don’t think it’s grit in there. Any ideas would be great.

    David Strong

    April 17, 2010 at 7:24 am

    • It sounds counter-intuitive, but try wetting the o-ring first with some silicone lube or DOT fluid. Also make sure to tighten the caliper bolts each a little at a time, not one before the other.

      SteveUK MTB

      April 17, 2010 at 7:55 am

  2. any additional tips for getting a piston to pop out?

    having some issues with one of them…

    shouldn’t be TOO stuck as this was just rebuilt, but seems o be a little jammed now. does move, just won’t come out.


    May 6, 2011 at 6:00 am

    • Are you trying to completely extract the piston, or will it just not extend? There’s not really anything for the piston to catch on if it’s straight in the bore, so the only thing that would impede it would be if it were twisted. There may, I suppose, be an air lock between the seal and the piston which would require a definite action to pop the piston out; but this really shouldn’t be too difficult to overcome. A clearer description of the problem would be helpful. Does the piston get so far then stop? Would you say that the internal end of piston has made it at least as far seal? Could it be twisting when it gets to this point and jamming itself in the seal aperture?

      If it’s just refusing to extend properly, then the silicone lube process I detail at the start of the guide should sort you out.

      SteveUK MTB

      May 6, 2011 at 11:39 am

  3. hi im trying to rebuild mine too,how does the pistion come out and when could i get the seals from ?many thanks phil


    May 8, 2012 at 5:00 pm

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