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Brake master cylinder, rebuild or replace?

fg1972

CB750 Enthusiast
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Hello again,
My front was a bit blocked up with crud but managed to get it free with compressed air up the line and working enough to allow me to remove all the pistons from the calipers. Now that I’ve fully dismantled the master cylinder, I could easily clean it up in a sand blaster then buy a new rebuild kit and new reservoir as mine is a translucent white plastic one that has deteriorated somewhat. The cost of the rebuild kit and reservoir is about triple the cost of buying a new aftermarket complete front brake unit. What are people’s thoughts on those aftermarket units from China? My rear also doesn’t work so I’ll be probably looking at a similar challenge when I get to that.
 
I wouldn't sand blast it. I imagine you won't get ALL the sand out, but maybe.
I went with an aftermarket master cylinder. So far so good. I know, it's not original, but mine isn't going to be in any concourse, nor will it be a collector as it isn't now.
 
I've used Chinabay MCs, and found that they must be disassembled, cleaned thoroughly, and reassembled properly with brake fluid lubing the assembly. Don't be surprised at the stuff that comes out when you clean.
 
My thoughts zactly................!!!

I'm intending on ordering after market MC in a bit, cause both of my fours front brakes are locking up.

I've rebuilt the calipers, but no the MCs.
 
How did you get the piston out? Mine is frozen pretty good, been sitting for many years.I'm soaking it in PB Blaster, not sure what else to do. If I get my master cylinder rebuilt I'll try using the brake itself to move the piston.

DC
 
Rig a grease gun fitting to caliper and use the gun to remove the piston. Will remove ones the M/C won't touch.

Have a care about it, at 5000 psi the grease gun will remove fingers if you get them in the wrong place at the wrong time.
 
Rig a grease gun fitting to caliper and use the gun to remove the piston. Will remove ones the M/C won't touch.

Have a care about it, at 5000 psi the grease gun will remove fingers if you get them in the wrong place at the wrong time.

Ditto, you can even scoop the grease back into a can and reuse!


 
The '81 had two calipers. One came out by using my small tire inflator forcing air into the fitting. Wouldn't work on the other one. Used a larger compressor the same way...blew it right out easily.
 
A mechanic made this for me. Piston stuck on a caliper from a '79 750. Grease gun pumped the piston out easily. IMG_0419.jpg
 
My thoughts zactly................!!!

I'm intending on ordering after market MC in a bit, cause both of my fours front brakes are locking up.

I've rebuilt the calipers, but no the MCs.
Locking up is usually caused by the tiny hole (some call it spluge hole) in the MC. I clean them with a high "E" electric guitar string chucked in a pin vice. Ancient collapsed lines can also cause it.
 
Mine came with an aftermarket master cylinder attached, but had the original. Cleaned the reservoir and bore. Ordered a new rebuild kit, and installed. Rebuilt the caliper and installed that too. Connected the brake line, and began pumping the brake lever. Watched the air bubbles rise up into the reservoir, and after pumping for about 15 minutes, could not get pressure. Before chucking the rebuilt original MC, I attached a vacuum bleeder to the caliper, and only then was I able to get pressure to the brakes. Not sure why.
 
Mine came with an aftermarket master cylinder attached, but had the original. Cleaned the reservoir and bore. Ordered a new rebuild kit, and installed. Rebuilt the caliper and installed that too. Connected the brake line, and began pumping the brake lever. Watched the air bubbles rise up into the reservoir, and after pumping for about 15 minutes, could not get pressure. Before chucking the rebuilt original MC, I attached a vacuum bleeder to the caliper, and only then was I able to get pressure to the brakes. Not sure why.
You usually need to bleed the MC by itself by holding a gloved thumb over the output until it is pumping fluid well, then quickly hooking up the banjo. Messy, but I've found it often necessary. Lost of protective plastic and paper towels for absorbing. Isopropyl alcohol works well to clean it off concrete garage floors. I hate brakes.
 
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