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CB750K Master Cylinder Bore

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Lobo:
Can anybody verify the standard piston diameter here please… my manuals suggest 14mm, but within the reservoir suggests 12mm. Furthermore, prior to any knowledge on this topic I simply ordered a Yamiya service kit and fitted it… without any dramas. Sadly I’ve binned the old piston and so can’t easily measure / verify the bore now.

I ask because the front brake has a woody feel to it; at abouts 20 mm (lever ) ball travel it’s biting, and within a very short further pull I’m at max effort.

I’m pretty sure the reservoir is original, and as stated, has just had new internals. The lines are OEM, ie NOT the braided type. I’m using sintered pads and the stopping performance is fine. To clarify, I’d just like a bit more travel / feel within the system.

Bryanj:
That was a common complaint of testers back in the 70's. If the lever is solid and the brake works plus all the pivots free and greased, thats what it is

Lobo:
Thanks Bryan; good to know. Yep, EVERYTHING about the brake system is tickety-boo so I guess I’m stuck with it.  Cheers.

K2-K6:
As noted above, that does sound like everything is in order for one of these brake system.

Honda were very, very conservative with these early production disc brakes (you can understand why, along with them being almost impeccably reliable from a component point of view) but do have as a consequence a fairly dull response to lever pressure.

Bore I've got from manual as 14mm and same as you Simon. Don't think the 12 in the casting refers to piston bore (could be wrong) you'd need to pull the piston out to really understand but looks all ok as is really. This gives a hydraulic ratio of 7.44 advantage to the lever which is pretty low by comparison to new system.

I feel that the sintered and ceramic pad materials need slightly higher line pressure to feel good, and sometimes experience will give opinion as you find that they feel wooden. Certainly don't have a very quick "grab" on application is routine comment, especially when compared to current systems in use.

I believe they can benefit from proscribed running to optimise  bedding in though to make best use of a well setup system. 50 miles of general use should get the pads something like flat/topographically mated to the disc in use, followed by a higher range heat cycle to get both disc and pads working well. That would entail riding with brake reasonably applied (run in third gear to give load) to hold pace down for about a mile to get the disc properly hot and enough to easily sizzle water if contacted. If hot enough at this point, don't stop the bike but just ride to let it cool brake to lower temp now.
Usually you'll feel the bite changing as you put more heat into the brake while running it, that's a good signal it's getting where you want it to be. This is especially true if the disc has been cleaned with brake cleaner or left around in a workshop for a while. Don't clean the disc again afterwards else you'll need to do this again.

Something else that can be worthwhile, the little hole in mastercylinder reservoir that feeds the piston/bore with fluid, deburr that entry point possibly with a hand used decent multi blade counter sync to reduce obstruction there. That's a production foible and shouldn't ideally be there.  It affects the system as you release the brake pressure with any unwanted resistance at that point causing the caliper piston to be withdrawn further that optimum. It's a small detail but can offer improvement in marginal feel and brake system response. Ideally would be a miniature bell mouth that's polished to improve entrance flow.

Lobo:
…thanks Nigel; will try the higher range heat cycle, but put the deburr on hold as it sounds like a thorough flush subsequently needed, even assuming metallic waste bits don't damage the (all new) seals.

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