Q&A

Classic Motorcycle Mechanics - - CONTENTS - James Rob­bins

Mark Hay­cock with a page of tips.

Q:I am the owner of a Honda MTX200 RII, which is a Ja­panese im­port but es­sen­tially the same as the 125 model. When I bought the bike, de­spite buy­ing from a dealer who said he had ser­viced it, the brake fluid in the master cylin­der was like jelly, so I bought a re­build kit for the master cylin­der and the front caliper. The bike has a twin pis­ton front caliper of a slid­ing de­sign with two pis­tons on one side. The pis­tons are made of plas­tic. The caliper had a lit­tle cor­ro­sion in the seal grooves, which I cleaned out and re­placed the seals, paint­ing on a lit­tle cor­ro­sion block grease in the grooves as per the ad­vice from Ralph Fer­rand in the magazine. After re­build­ing the brakes and bleed­ing them, I now find they bind slightly to the ex­tent that if I ride up the road and don’t use the front brake at all, after around half a mile the disc is too hot to touch. Fol­low­ing this I re­built the caliper again, dou­ble check­ing ev­ery­thing and us­ing new seals of OE qual­ity from David Sil­ver spares. I cleaned out the grooves again but with­out any cor­ro­sion block grease. It’s ex­actly the same and I can’t see why its bind­ing. The pis­tons are quite tight to get back in their bores, but the small amount of cor­ro­sion in the seal groove on one pis­ton has never been an is­sue on other bikes I have worked on. A new caliper body is un­avail­able new (even if I could af­ford it) and a sec­ond-hand one is likely to be the same as mine. Would you sug­gest buy­ing new pis­tons? Or is there some­one who can re­furb my caliper?

A:I think the is­sue you are fac­ing is prob­a­bly not be­ing caused by the pis­tons but in­stead to do with the slid­ing mech­a­nism. I say mech­a­nism, but all it con­sists of is a cou­ple of pins screwed into the mount­ing bracket, one of which slides di­rectly within the caliper body with the other slid­ing within a bush that, in turn, fits in a hole in the caliper. You need to dis­man­tle this to check for cor­ro­sion and if you find any, sim­ply re­place all the parts, ex­cept the caliper body, as they are quite cheap. Use your anti-cor­ro­sion grease (though I must say I al­ways get good ser­vice from or­di­nary sil­i­cone grease) to en­sure that the mech­a­nism does not stick again. There is one other pos­si­bil­ity and that is that some­thing might have gone wrong with the master cylin­der, to the ex­tent that it is hold­ing pres­sure within the hy­draulic sys­tem even though the lever has been re­leased. You did not men­tion the master cylin­der other than to say it con­tained jelly rather than fluid, so I as­sume you have over­hauled it? If not, a gen­uine Honda over­haul kit (part num­ber 45530-kj1702i) is avail­able, though rather ex­pen­sive at around £60.

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