Clarke skeletal/cheap hydraulic disc problem

[The Family Cyclist]

I'm trying to help a friend get their bike sorted.

His bike has skeletal disc brakes which don't seem to be sold any more. The rear one drags horribly. I cracked the reservoir cover off and pushed the piston back in but it comes out and over a few pumps gets to the stage where the wheel won't spin more then a couple of turns

From what I can understand it only has one moving piston but unlike my very old hopes nothing to spring the pads apart again.

Just wondering if anyone has any advice on these. As far as I can tell my options are

Take the piston out and clean and reinsert with same seal

Try and source replacement seal which looks difficult, have emailed clarkes

Try aligning the disc again but I don't think that's the problem as they haven't been moved. The bike hasn't been used for ages which makes me suspect it could be gummed up.

Other option is another cheap caliper

[Brucey]

clarks M2 brakes are cheap on e-bay; often they can be had for less than £40 a set new, and 'for parts only' items for buttons, used.

If you want to repair the extant caliper then you need to try and work out what is going on. The brake piston will normally retract partly because of the MC 'sucking' if you like but also because of 'seal rotation', i.e. the seal deforms in a highly specific way that allows the piston to retract slightly when the pressure is released. This approach is also used on car and motorcycle disc brake calipers designs.  Various things can interfere with piston retraction, including

a) that the piston is binding in the caliper bore; try lubricating it with the brake fluid
b) that the seal is damaged or badly seated; the seal groove needs to allow the seal to move and if it is filled up with grot/corrosion it won't allow the seal to rotate
c) that the seal has swollen and again can't move in the groove. This commonly happens if the wrong brake fluid is used, or a general purpose lubricant is used on the caliper and penetrates as far as the piston seal.
d) that the hydraulic system is badly configured so that the pressure isn't released in the working part of the circuit when the lever is released. For example if the system is an 'open' type, air trapped in the system or a MC compensation port that doesn't open can cause pistons not to retract properly.


In most bike shops they don't bother messing about with cheap hydraulic brakes; at their labour rates it is more cost-effective to replace them.  Valuing your own time is different to that of course.

It is possible to buy most kinds of hydraulic seal if you look hard enough (there are usually identifying marks on the seal itself) , but it is often a lot of effort for little reward.

cheers

[The Family Cyclist]

My labour is free, don't mind a bit of fettling. I'm not prepared to stick a huge amount into trying to source a seal. Debating whether to take the piston out and clean. The fluid will be from new so won't have been the wrong type. My friend isn't a fettling sort and certainly wouldn't have the tiny torx bit needed for the reservoir cap

[Brucey]

FWIW I am told the 'Eriks' are likely to have most types of seal, but I've not BTDT.  Looking at their website, they do seem to list what they are likely to have, whereas a lot of traditional seal suppliers more often don't, (and then get grumpy when folk ask them if they have a such -and-such.... )

cheers

[Brucey]

BTW maybe I have never seen the exact model you have in front of you but every 'skeletal' brake I have seen has the usual layout, i.e. hose connection one side of the caliper body, bleed port on the other, two pistons, using DOT4 fluid.   Fluid type aside, they look like pretty fair copies of Magura Marta / louise brakes.


skeletal


If it looks like there is one moving piston e.g. because the other piston is seized, you could have more problems than you might suppose.

FWIW if one piston is seized, you can usually pack the caliper body out in such a way as one piston is blocked, but the other isn't, and apply brake pressure. If a seized piston doesn't move given this treatment, then the chances of saving the brake (esp with no new seals on hand) are pretty slim.

cheers

[Torslanda]

pedalbits on eBay are currently selling Shimano 315 brakes for around 38 quid a pair. Complete.

It really isn't worth buggering about. Change them.

[The Family Cyclist]

Thanks. Clarkes haven't replied unsurprisingly. I'll see what my friend wants to do.