Front master cylinder rebuild

SomersetChris

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Joined
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Total Posts
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Location
Clevedon, Somerset, England
I've just spent a couple of hours trying to strip the front brake master cylinder - who designed that?
Of course the piston was seized in there, but the clip holding the remains of the rubber boot & retaining circlip were the biggest problems - only access via the hole where the lever pushes the piston.
Circlip pliers either to big for the hole or too short to reach, so a long time cursing with long thin screwdrivers until the circlip gave up.
Inside the reservoir there's "10" stamped but the piston (& bore) are 14mm - any ideas if the 10 is significant?
IMG_20251104_193039.jpg

Then I had to beat the piston out, using a long thin punch from the hose connection end. May try a reamer (silicon carbide brush) fitted to my drill to clean the bore.
I've done that before with a BMW cylinder - but that was cast iron.Next on the list is the caliper - seized of course.
I think I prefer the drum brakes on my K1!
 
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Usually a custom set of C clip pliers are needed. When they are frozen down in there it is never an easy task to remove that C Clip. The #10 means nothing in relation to the rebuild kit or process.
That very tiny spooge hole down in the one of the two holes in your picture must be clean. A small thin piece of wire maybe needed to unplug if it is closed up with old crystalized junk. That is the pressure return hole in the system and if not clean you will never bleed the brakes correctly.
If you have an Ultrasonic cleaner and the piston is frozen in the caliper that will assist in removing the piston if the cleaner has a heater setting. Get it hot and run it through a number of cycles. I personally use compressed air into the brake line connection hole on the caliper. You need to be careful with rags to catch the fluid and piston as it is expelled from the bore. The piston must be treated as a potential UFO, if not cushioned upon its release :unsure:. Watch that your fingers are away from the piston path of exit.
When the brake master is still operational you can use it with the brake fluid in the system and hose still connected to push the caliper piston out safely. You don't have that option this time it appears.
 
SomersetChris said:
I've just spent a couple of hours trying to strip the front brake master cylinder - who designed that?
Of course the piston was seized in there, but the clip holding the remains of the rubber boot & retaining circlip were the biggest problems - only access via the hole where the lever pushes the piston.
Circlip pliers either to big for the hole or too short to reach, so a long time cursing with long thin screwdrivers until the circlip gave up.
Inside the reservoir there's "10" stamped but the piston (& bore) are 14mm - any ideas if the 10 is significant?
View attachment 51977

Then I had to beat the piston out, using a long thin punch from the hose connection end. May try a reamer (silicon carbide brush) fitted to my drill to clean the bore.
I've done that before with a BMW cylinder - but that was cast iron.Next on the list is the caliper - seized of course.
I think I prefer the drum brakes on my K1!
Click to expand...
I've thrown in the towel & bought a new aftermarket master cylinder & caliper, after deciding that the originals are beyond redemption! Also had new braided brake hoses made - I'm not buying new rubber ones just to appear original, I prefer decent brakes. Slow, steady progress. IMG_20260421_184131.jpgm cyl 2.jpg
 
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SomersetChris said:
oh & that nasty rigid nylon (original?) Cheng Shin tyre has gone - I fitted a new Michelin Road Classic after cutting off the CS with a hacksaw!
Click to expand...
AFAIK it would not be original, Honda used mostly Bridgestone and Dunlop during that period and never Cheng Shin.
 
Looks like your adjustment screw broke off. It should be replaced and have a lock nut on there to keep the screw from coming undone.

1778809367976.png
 
Piping in here mostly uselessly, but tasteful and wise choices were made. Enjoy working on it! That #10 on the original part is likely a mold/stamp number, by the way.
 
That brake adjuster screw is available, the instructions on making the adjustment are in the FSM, after rebuilding my caliper I set mine with slightly more gap and it working great
 
ancientdad said:
AFAIK it would not be original, Honda used mostly Bridgestone and Dunlop during that period and never Cheng Shin.
Click to expand...
I just wondered as it had that 70s look about it!
I do remember Bridgestones being fitted to Jap bikes back then & they were slippery too - but now they are excellent rubber.
Hope the Michelin is good as I could not find an Avon Roadrider to match the rear - no longer in production since being taken over, I gather.
 
SomersetChris said:
I just wondered as it had that 70s look about it!
I do remember Bridgestones being fitted to Jap bikes back then & they were slippery too - but now they are excellent rubber.
Hope the Michelin is good as I could not find an Avon Roadrider to match the rear - no longer in production since being taken over, I gather.
Click to expand...
Yes, Cheng Shins do have that '70s look and they were around then as well but when I first saw them back then I was skeptical. I ended up buying one or two for various reasons over the years and they were decent in my experience.
 
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