Master Cylinder Rebuild

[Mystic Panama Red]

This question is regarding a handlebar mounted master cylinder on an '84 R 80 G/S. I'm rebuilding the master on this bike that hasn't run in at least 6 years. It had leaked, and was full of lots of gunk probably due to the DOT 4 fluid absorbing water and then just sitting. I've totally disassembled it, and cleaned all parts. I've been referencing the article in my dear old Clymer book (the one with the beautiful R75/5 on the cover with the silver tank that is missing its rubber pads). Where was I? Oh yeah. They have great instructions on doing this master cylinder. They call it a Type C, and have a great set of pictures running through the procedure

There's a question coming before long. Hang in there.

If you hold the main body in your hand and look down where the fluid would enter from the reservoir into the cylinder where the piston is, are there always TWO holes there for that fluid to pass down into the cylinder? The Clymer book (page 627, figure 133) shows two and tells you to make sure "the passages on the bottom of the body are clear."

My master cylinder clearly has the hole on the left, but it seems like the other one was only drilled part way down, but not all the way through . . . weird. I kept thinking there was a much smaller hole down the middle of it, but the tiny pick I'm using isn't finding that it is drilled through at all. I'm wondering if that second hole might only be used for bikes with dual front brakes.

Anybody know for sure? Thanks in advance for any help.

[DanielMc]

My guess is that the hole is blocked and will need clearing out if the M/C is to work.

[ME 109]

Check that bore for corrosion/pitting.
No point putting a new kit in there if it's rough.

[lrz]

I've been doing the same thing. Yesterday I dealt w/ the undertank variety for the first time, BIG HASSLE compared to the barmount set up.

Compressed air is a good tool for clearing passages of all sizes w/ out marring or damaging anything.

I also stripped the caliper bare figuring why not tackle the entire system all at once & be done with it. Now if only parts could be ordered on a Sunday. Or a Monday...

[Mystic Panama Red]

ME 109, you're right. After soaking the metal body all night in mineral spirits, I took a good look down the bore with a flashlight. There is enough corrosion on the bore itself to make me throw in the towel on a rebuild, and concede to buying a new master cylinder for it. There is no sense in spending half the amount of a new one to rebuild this one if it might leak again soon.

I've always figured the brakes are the most important part of any vehicle. If the engine won't run you are inconvenienced. If the brakes won't stop you the consequences can be much worse . . .

[Major Softie]

I've always figured the brakes are the most important part of any vehicle. If the engine won't run you are inconvenienced. If the brakes won't stop you the consequences can be much worse . . .

Yep. Tough to choose between tires and brakes as your most important components. Lets just call it a draw.