Mark Haycock sorts the front brake caliper. Ish!
So, Mark found that some parts of his K2 were from a later model – which means sorting the brakes can be a bind. Literally!
Imentioned last time that the K2’s front brake (Photo 1) was evidently not original, appearing to have been taken from a CB750F. It did actually work a bit, in that there was some resistance to the lever’s movement though it did pull right back to the handlebar without too much effort. Assuming it was not completely shot (and bearing in mind I am not particularly fussed about making a K2 museum piece) I intended to use what I had. It was easy enough to separate the two halves of the caliper, while it was still mounted on the front fork which makes sense as it is easier to undo the bolts that way. With the hydraulic system still intact, I could eventually pump the piston out of its caliper half (Photo 2). It is best to check the working part of a component before worrying about its external appearance, and the good news was that the cylinder surface seemed okay, just needing a bit of treatment with a miniature wire brush which produced quite a bit of dust (Photo 3) but left the surface serviceable. In truth, the only critical part of the cylindrical surface is the groove for the rubber sealing ring and this was fine. You can see in Photo 1 that the paint on the outside surface would just fall off as soon as I touched it, or so it seemed, but that was not quite true I am afraid. I checked in my stock and I had some safer-type paint stripper (Photo 4) which proved to be so safe it did nothing at all, even after several applications! Conventional dichloromethane stripper did a better job, but it is important to be aware that you need to take care with it as it is not one of the safer solvents. Try wiping it on using your fingers and see what I mean. It is also believed to be carcinogenic and it destroys the ozone layer – but it does take paint off. Your choice! Now, you are probably wondering where I sent the bits off to be repainted black but… why is it painted anyway? After all, you need to take care not to spill brake fluid on painted surfaces so surely I would be better to leave it as it was? A bit of treatment with a Scotch-brite wheel left a pretty reasonable surface to match the fork cases we saw last time – certainly a whole lot better than it had been (Photo 5). All I have to do is give it an occasional polish and it will be okay. The piston was, as usual, pitted and rusty so it would have to be replaced, along with the bleed nipple which was also corroded but not actually rusted in place. Unfortunately, the replacements I bought did not fit. The piston was too small (Photo 6) and the nipple too big (Photo 7). Oh… wait a minute… I had bought them along with a few other bits and forgotten I should have specified CB750F not CB750K2. I got the correct parts easily enough, though the nipple, in stainless
steel, while very keenly priced was not properly made and had to be sent back. The fault was that the screw thread had not been correctly formed because the blank had been too small in diameter (no doubt because it is easier to cut) but for this application that is no good as the fit was much too loose and consequently it would leak when bleeding the brake. It is interesting, by the way, that Honda specified a larger diameter piston and smaller diameter bleed nipple for the later caliper. The master cylinder internal diameter remained the same so presumably the later type caliper can produce a greater braking effort, at the expense of extra lever movement. Why the smaller nipple worked better, I do not know. The sealing arrangement for the cylinder is very straightforward, the standard set-up consisting of a simple square section rubber ring with no dust seal. I say that because the piston I acquired was not quite like that. In Photo 8 we see to the left the standard genuine Honda K2 set-up with the smaller hollow piston and single rubber seal. To the right is the nonstandard hollow CB750F piston with a plastic insert, to convert it into a closed piston which is fitted as standard. There is also a narrow-section ring which I assume is to act as a dust seal, but it is not shown in the F parts book and nor could I see where it was meant to fit so I left it out. The pads fitted were practically unworn but one had what appeared to be a piece of steel swarf embedded in it (Photo 9, left-hand one) so I did replace them with my usual EBC choice. I must say though, that I was expecting new EBC pads to work well in the TX500 front brake, but I cannot say that the result has been desperately effective though that may well be a weakness of the design rather than the pads themselves. It would have been good if the little hole to accommodate the split-pin had been drilled big enough, but I could always do it myself! (Photo 10). The caliper is mounted on a pivoting arm and this assembly was modified for the later-type forks fitted to my bike. One difference is that the spacing between the caliper mounting bolts is increased which no doubt increases resistance to twisting, and the pivot appears to be stronger. It also incorporates a strange little mudguard (Photo 11) which looks as though it is meant to keep the disc dry but really is to provide a shield against spray from the disc. You should note that two O-rings are fitted in the pivot to stop water from finding its way inside (Photo 12). I use silicone grease here as it is very water resistant. One tiny problem was that after I installed the caliper on the fork leg, I found that the brake was jammed. This, it turned out, was caused by my having put in the plastic insert the wrong way round – easily corrected!
The master cylinder needed a complete overhaul but I was fortunate that the visible parts were in pretty good condition (Photo 13). If the seal leaks, the brake fluid attracts water which rusts the circlip in place and it can be impossible to remove. The inside surface of the cylinder needed a bit of cleaning up and I was not too impressed with a special hone I bought for the purpose, so this time I put together a home-made hone consisting of a bit of wire bent in half and strips of wet and dry paper (Photo 14) mounted in an electric drill. This worked rather better than the hone; naturally you start with a coarse grade working through to a suitably fine grade in stages. I did splash out on a genuine Honda overhaul kit (Photo 15) which on reflection I do think is probably a safer bet. The external parts of the master cylinder were interesting in that I could see underneath the contrast between the original black and the current colour of the paint (Photo 16). This does not show in use as the black bits are covered up (which is why they are still black) and I don’t mind about the fading as to me it looks okay and it simply reflects the age of the bike, which is not something I am anxious to disguise. Less okay was the bent brake lever (Photo 17) which I replaced with a new genuine Honda rather than pattern part as it was not too much more. I even bought the little rubber cap to fit on the ball end. The clutch lever was in a similar state and I think this damage is probably caused by carelessness in transporting the bikes over to the UK. One subtle point about the lever is that when fitting it there is an oval plate underneath the cylinder, the purpose of which is not quite clear. It is actually to hold in place a little rubber cylinder (Photo 18) the purpose of which appears to be to act as a buffer to stop the lever making an annoying clack noise if it is released quickly. This is a Honda, after all. Next month I try and sort out the CB750 K2’s back end. Wish me luck!