Master Cylinder Seems Clogged

I've got a '76 XS750 Triple, been slowly cleaning her up since the fall. The brakes were sticking so I pulled the system a few months ago and finally got some more time to work on it. MC was building pressure (that's how I got the pistons out of the calipers) but now it seems clogged after sitting dry for some time.

Used a compressor to blow some air through it, but it's still not moving any brake fluid. Ideas? I read somewhere that brake fluid crystalizes and can be a PITA to clean? Pulled the reservoir off the housing and used some simple green, didn't help.

Thanks in advance!

You need to disassemble the MC plunger and clean all the internals. Piston is probably seized. But that small c-clip is a bitch to remove.

See above. once you put it back together, make sure you brake lines and banjo bolts are good and you have new washers...

That's what I figured, was trying to avoid it. Any recommendations on cleaning the body up? I was going at it with a die grinder (wire brush) but it barely made a dent in the corrosion.

I just ran into this same problem. Air pressure wouldn't budge the piston one bit. The exterior was badly rusted and pitted, but the inside wasn't as bad. The reason it was jammed was because the old brake fluid turned into a solid, probably the same as yours. Here is how I was able to get the piston out, and rebuild it.


First it was left to soak overnight in a acid based degreaser. It helped some, and made the caked up brake fluid on the outside look like a fungus. Wire brush took it off easily after it soaked. Without doing this, I don't think the cir clip would have come off.



Then the rear master was hooked up to a working front master cylinder, and the reservoir hole was plugged with a bolt.



After bleeding the air out of the lines, and building up pressure, you could push the lever as hard as possible, and then feel the piston move some under the pressure. Then build up the pressure again and repeat until it is far enough to pull out with pliers, or vice grips. You may need to add more fluid to the reservoir during the process. Look at that nasty junk!



The spring was still stuck, but it came out with some long needle pliers. To polish the bore I used a gun cleaning kit hooked up to a drill, because dremel bits were not long enough. It worked fairly well. I also had to use a small drill bit to make sure all the crud was out of the two small holes that lead from the reservoir to the piston cylinder.

Wow! Thanks for the info and pictures man. I was thinking of soaking it in carb dip, sounds like I was on the right track

redstripe89 said:

Wow! Thanks for the info and pictures man. I was thinking of soaking it in carb dip, sounds like I was on the right track

Click to expand...

The longer you can let it soak the better. I would like to have tried CLR if I had some at the time, since that eats rust too.

Wow, I can't believe how much came out of that thing.