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Front brake master cylinder piston replacement

mnvkngs65

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Rehabbing my newly acquired 1973 CB 750. Ordered a new front brake master cylinder rebuild kit for a 73 and the piston is too narrow.
I believe the master cylinder on the bike is from a Goldwing based on the shape of the front of the brake lever? (the front brake was converted to 2 discs at some point - not sure if that is why the master cylinder was changed)?
The piston I need to replace is approximately 18mm wide at the center and approximately 49mm long. I can't find where to order a rebuild kit based on the piston size.
Any help would be appreciated.

Thanks!
 
Photos of current piston and brake lever attached. IMG_20190820_083103738_HDR.jpg
 
Rebuilt the master cylinder. It doesn't seem to build pressure and a small amount of brake fluid leaks out around the dust cover at the brake lever (like it was probably doing before based on the crud noted on the brake lever - see pic above).
Broke it down again last night to make sure seal and parts were properly installed. Confirmed and put back together. Piston and seal fits very snug as expected. Still won't pump brake fluid (or a very minimal amount drips ) out of the master cylinder with repeated pumping of brake lever. Everything was cleaned meticulously and blown out. No obvious debris or irregularities in the master cylinder. Going CRAZY with this $&_#@+ thing. Do I need to buy a new master cylinder? Any help appreciated.
 
You MUST hone the ID out and then look for low spots that bypass to leak. Clean alone will not work. After honed you can look at the bore with strong light in the other end and 99% chance a used one will show at least some corrosion low to leak, up to you to decide how bad it is then. Almost impossible not to have some corrosion on the bottom side as the cylinder mounts to handlebar. The bowl of part must have both the bigger hole and the small pin hole open as well.

If the casting will not seal then you need an entire new one.
 
Grrrrr. Ok will take it apart again, hone the ID and make sure the inlet holes in the bowl are clean and functional. Thanks again.
 
Honed and cleaned the ID and reassembled. Seal is not leaking now but still very little and inconsistent fluid comes out the lines. Lines are clean and easily drain fluid when disassembled from master cylinder. Thoughts?
 
Some gravity feed the fluid to let it run down the lines. The problem is that the total displacement of the cylinder is not enough to completely push all air out because the lines are too long for the volume the cylinder puts out. The result is that air purges to partway down the lines then you run out of cylinder travel and the air then floats back up high. If the cylinder is now positively pumping it will take a while to build up enough fluid in lines to overpower the air. The cylinder needed to be taken to a vise or similar and then you fill it up and hold finger over the outlet and cap off and slowly pump it. The cylinder fills and the at some point you will not be able to hold finger over outlet as it will be blowing fluid out of it and you cannot stop that. Once cylinder is positively pumping then you attach it back up and pump away. Some use a MityVac hand vacuum pump to draw air out the caliper, they pull quite a bit more volume out with each hand pump and it accumulates too.
 
It's working! However I am unable to remove one of the pistons. I'm too nervous to to push the brake too far. should I soak it in PB blaster in an attempt to free it up? Thanks.
 
Uh, a little late for that, you are trying to mix petroleum distillates with brake fluid, one of the big brake no-nos. Petro exposure fails brake hydro parts. The caliper should have been rebuilt before trying to fluid it up. You won't be able to get all PB out and not the way you free it up anyway. Piston has to come out with brake fluid pressure (meaning it works) or tear caliper apart to free the corrosion out of it.

Too nervous to move the brake? If it won't then it won't work on bike either, we're talking your safety there.

If the piston is truly super stuck then you figure out a way to plumb a grease gun to the fitting and pump the piston out, not moving then says new caliper time. If piston comes completely out the whole caliper gets rebuilt. You can use compressed air but incompetence there can kill you when the piston shoots out like a cannon shell. Put a shop towel in over piston wadded up.
 
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