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So, I bought a new master cylinder for my '37 115c. Then I painted it, put it on my frame, and it sat outside for about a month while I was finishing the chassis. The cap was on.

The problem is, I forgot that the cap is vented, so some water got in and the inside is rusty. I have never put fluid in the master cylinder.

My thought was to disassemble it, inspect the bore, clean as required, and reassemble, but I don't know what to do about the rest of the master cylinder interior.

Any thoughts on how to clean the rest of the master cylinder?

Dear Ken:

Your new unit should have been assembled using what is called "brake assembly lube" which is non-reactive with the rubber parts.

I assume by rusty you mean the inside of the reservoir. I have run into problems with moisture from garden hoses and what not getting past the vents - even in modern cars - so hosing off motors is not a good idea. Unless you cover things with plastic bags.

Quite frankly I would contact the rebuilder and ask if they will replace because your unit rusted on the shelf. Unless it was sitting outside in the weather in which case you are stuck.

The outside can be cleaned with metal etch and a rag. Ditto the inside, but it must then be washed with detergent and water and then washed with alcohol, dried, and reassembled using brake fluid as lube.

If there is any pitting in the bore then you need another unit because the unit could bypass causing the pedal to go to the floor.

Good luck