Rear master cylinder

[cbxdragbike]

I am rebuilding a rear master cylinder, there are 2 little holes for the brake fluid to get into/ out of the piston area. The smaller one seems to be partially plugged. Does anyone know what size this hole is supposed to be and is it OK to drill it out.



Mark

msproul@jove.rutgers.edu

[Mike Nixon]

Don't drill it--poke it, with a piece of wire.

[cbxdragbike]

I have been trying and I cant get it to poke through. I have another unit that I can. I have tried several different size wires. On both units the hole starts out big enough to get a very small drill bit through and then narrows down. On the bad unit, I cant get anything through. I have tried squiriting WD-40, Brake cleaner, soaking it in carb cleaner and in kerosene.



Mark

[Dave Ditner]

The small hole is the compensating port. It's gotta be clean. Alcohol should work as a solvent. Put it in a jar full and let it soak. Should at least soften it up so you can blow it out with an air hose. You don't want to drill it. You could raise a burr and if you do you will cut the lip of the cylinder seal. That toasts it. The forward edge of that hole is where the seal is generated and you begin to build pressure.

[Passx]

Mark,



One thing that needs to said about your cleaning products choices, everything you listed except the brake cleaner will ruin your new seals, they are all except the brake cleaner not compatible with epdm rubber and will cause it to swell and fall apart. It only takes a very small quantity (a drop or less)for this to happen. Either use only a recommended brake cleaner or alcohol. Conversely you can't use brake fluid products with mineral oil type seals (products that use engine, transmission oils, kerosene, gasoline, etc,etc). Good Luck



Steve P. #5220

[Tom Neimeyer]

I use the smallest size "pin drill" #80, which is about 0.013. That size is actually smaller than the hole so it does not enlarge it or leave a burr, just cuts through the dried brake fluid. The pin drill is finger powered so it is pretty safe to use.



Since this cleaning operation is usually part of a rebuild, I then use 600 grit to clean the inside bore.



Tom

[Tom Neimeyer]

I meant to add, like Mike N said, "poke it" I usually just press/slightly turn the very small pin drill into the hole. Once it goes through I use a spray carb cleaner to complete the cleaning. If you need a small set of Pin Drills, E-bay has several types/brands. Do a search for both "pin drill" and "small drill bits". They range from $7 to $15. Or you can do like I did my first set, go to a Snap On dealer and pay $35. Second set from E-bay.



YMMV, Tom

[Rick Pope]

[quote="Tom Neimeyer"]I meant to add, like Mike N said, "poke it" I usually just press/slightly turn the very small pin drill into the hole.



Some of us are too tight to buy pin drills, so we use a short piece of electrical wire. 12ga works well. Strip the wire as though you want to make aconnection, then just use one strand of the wire while holding the unstripped portion. You can twirk it between your fingers like drill, and the copper won't raise a burr.

Like I said, some of use are cheap......

[Jeff Bennetts]

I also rebuild alot of carbs, so long ago I went down to to local music shop and bought a selection of guitar strings. The wire is incredibly strong, comes in many small diameters and cost a couple bucks a piece.



jb

[DC354]

Use air to clear out the holes. I did my front master and was wasting brake cleaner on it cleaning it little bit at a time until i finally figured out to just blast it.

Does anyone know where i could track down a rear master cylinder? I haven't had any luck whatsoever!
These guys seem to fit most of the old CB series. Think it might work on the cbx?
http://www.ebay.com/itm/MOTORCYCLE-DIRT ... 02&vxp=mtr