Cleaning crude from master cylinder bore?

I opened up the front master cylinder to replace the plunger on my 79 KZ1000ST's front master cylinder. Bleech!,goo to go in there. Washed in alcohol which got rid of the 30 years of sediment, but, there seems to be some hardened crude deposits on the inside of the cylinder bore. These are not coming off with an alcohol soak and I'm afraid to sand or scrub the cheap pot metal in case I make matters worse. Any good chemical product I can use to get that stuff out that won't destroy the paint on the out side of the cylinder?
This winter I'd like to redo the outside of the master cylinder with some surface treatment that's impervious to DOT 3 brake fluid. Any suggestions, I'd like to keep it stock black if possible?
Thanks upfront,
Ken

There may be better methods, but I've enjoyed success by carefully and gently wet-sanding the bores with ultra-fine wet-or-dry sand paper.

I call this "dressing the bore," which removes any high spots that may have resulted from pitting. Just leave the pits alone, which are hopefully benign enough to allow master cylinder operation without leaking fluid.

The rubber cups may be carefully cleaned, and if not torn, may sometimes be re-used satisfactorily and without leaking.

Good Fortune!

bottle brush and brake cleaner?

I used a small wheel cylinder hone on mine olny took two quick passes.

Went with your recomendation, the pitting and rust came clean and the cylinder seems to be working ok now. Still not great braking, think I'll do the calipers this winter and replace brake lines with braided.
ss^
Ken

They make Ball hones in the small brake cylinder size, say 16mm or 5/8". That's what I use. If it doesn't clean up then two choices, replace it or sleeve it. Then always use a new set of cups, buy the rebuild kit.
doing the caliper/s at the same time would be recommended, as well as a new brake line. these bikes are pretty old, and this is an area that would be overlooked on most used bikes.

DOT 3 VS DOT 4 Brake Fluid

I just flushed my brake lines with new DOT 3 fluid only to find out that the master cylinder has a leak. Instead of rebuilding the master cylinder, I bought a 'newer' used MC off of ebay, but it calls for DOT 4 instead of DOT 3! D'oh! Well, I've heard that 3 & 4 are very similar and I'm wondering how thoroughly I need to flush the DOT 3 fluid out of the brake lines? Can I just 'push' the DOT 3 fluid out by adding DOT 4 to the master cylinder and pump until I think I have all of the DOT 3 out of the lines? How would you do it?

...Can I just 'push' the DOT 3 fluid out by adding DOT 4 to the master cylinder and pump until I think I have all of the DOT 3 out of the lines? How would you do it?

YES. That method is just fine. Same for the routine "flushes" without requiring re-bleeding from scratch.

I use a rubber squeeze syringe to remove most of the existing reservoir fluid, but leaving the bottom ports covered with fluid so as to avoid air entering the line. Then fill with new fresh fluid, pump it through (always leaving the bottom ports covered with fluid so as to avoid air entering the line) and repeat refilling and pumping until the new fresh fluid exits the bleeder valve.

Patton

fxdl2051 wrote: Still not great braking...

Sounds like you might need to bleed them.