Rust in the Master Cylinder
 

Opened up my master cylinder cover to check the fluid level and came up upon this surprise. Rust on the wall of the front chamber. Nothing on the rear chamber. Car gets used about twice a week in nice weather. Not much in winter.

Any idea why rust is developing in the master cylinder and why just the front chamber?

Can this be cleaned out or am I looking at replacing the master cylinder? I assume at least a fluid change.


https://i.imgur.com/mmep5bM.jpg

You need to flush the brake fluid every couple years as brake fluid soaks up water. The synthetic fluid is better in that regard. If its an original master you can have it rebuilt.

I would just remove it, dump out the old fluid, sand down the rust / sandblast, etc. Then refill, bleed master off the car, then remount it.

If you want to keep that master cylinder for some reason, it can be cleaned, rebuilt and put back in service. Once removed, a diluted solution of muriatic acid will clean the rust right out. if the bore is rusted as well, it could be sleeved at a place like White Post Restoration if your that invested in keeping that MC. As far as why it is rusted in one chamber, most likely the rubber diaphragm under the cap had a crease, tear or pin hole that let air come into direct contact with the fluid and absorb the moisture from the air. The rear chamber was working properly. The air is supposed to enter the vent in the lid and push the diaphragm down slightly when braking or if the fluid level drops. Little to no outside air gets into the fluid area directly. That looks like a simple/common MC that could be replaced with a new one inexpensively.