SOHC.co.uk Forums > CB350/400
Brake master cylinder rebuild kit
Safo:
I attached a syringe to the bleed nipple on the caliper and sucked the fluid down whilst pumping the leaver, once fluid was in the system I just bled as normal, must add that was a 500 m/cyl
mattsz:
Regardless, I have a project ahead. I watched a Youtube "video" (mostly a narrated slide show) which shows the master cylinder rebuilder inserting the spring with end caps, piston (with rubber seal installed) and flat washer, then getting the circlip just into the cavity and pushing the whole works down into place with a deep socket, which pushes on the clip evenly until it snaps into its groove and everything is in place in one go.
My reality: even with lots of brake fluid lubrication, the rubber seal on the piston doesn't insert without getting caught and folding up backwards on its way into the cavity. I have to hold the piston in just the right spot against the spring pressure and use something small and gentle to push the lip of the seal into place (I used a red "wd-40" spray-type applicator straw). But then I can't let up on the pressure holding the piston in place to install the washer and circiip or the rubber seal pops back out again - and I can't reinsert it with the washer and circlip in position. I need four hands...
I removed the spring, re-inserted the piston and left it overnight - maybe sitting at rest, compressed into its working position will help make the seal easier to insert tomorrow... where I'll have some extra hands at work to assist me if need be...
K2-K6:
I find assembly of brake seals etc easier with rubber grease or silicone grease, the parts just offer less "snagging" and resistance in the situation you describe.
Prefer to leave it assembled with grease present on the outside rather than remaining brake fluid.
Deal with the bleeding through as already described above.
Have a close, very close, look at the bore where the seal catches to make sure any previous corrosion has not left it effectively undersized.
mattsz:
--- Quote from: K2-K6 on August 10, 2020, 09:18:45 AM ---I find assembly of brake seals etc easier with rubber grease or silicone grease, the parts just offer less "snagging" and resistance in the situation you describe.
--- End quote ---
Thanks - I assumed that having grease on parts that are exposed to brake fluid was something to be avoided. I don't have any "rubber" grease, but I do have silicone grease.
--- Quote ---Prefer to leave it assembled with grease present on the outside rather than remaining brake fluid.
--- End quote ---
Not sure what you mean by this - I was planning on putting some grease on the washer and circlips to help prevent them from rusting up down the road...
--- Quote ---Have a close, very close, look at the bore where the seal catches to make sure any previous corrosion has not left it effectively undersized.
--- End quote ---
I have done this to the best of my ability. I cleaned everything I could reach, then "honed" the cylinder with a small bit of cleaner-soaked 3M green scrubbing pad "trapped" on the end of a drill bit. When inserting the working end of the piston into the cylinder, it fits perfectly and moves freely as long as the piston is perfectly centered and in line with the cylinder - if it's at all crooked, the piston drags and binds if I try to insert it...
K2-K6:
"Thanks - I assumed that having grease on parts that are exposed to brake fluid was something to be avoided. I don't have any "rubber" grease, but I do have silicone grease."
It's fine to use either red rubber grease ( most systems would be assembled with this in production) it'll just dissipate when the system is filled with fluid, it's not a risk.
Silicone grease also is compatible with dot 4 brake fluid and won't compromise the integrity. Most of these system will run dot4 or dot5 which is silicone brake fluid.
The bore originally would have have a very small chamfer on the approach side of the clip rebate to facilitate the seal passing into the bore. It will often get left really sharpened and angular after corrosion takes place. It's safe to re establish this as the seal will never need to operate on this area once it's passed through.
To "scrape" it by hand you'd need to grind a tool out of an old hand file or similar to recut it. Or usea proprietary deburr tool to get in there. It's only a small amount but stops the seal peeling back as you are getting.
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