Rust again in master cylinder

[ward 57]

I just had my M/C rebuilt last year and all new flexible hoses and flushed. When doing my spring fluid checks the fluid in the M/C was already turning dark with rust. I'll siphon it out with a turkey baster & refill with new fluid. Hopefully it hasn't gotten any further. Why is it rusting again so soon?

[55blacktie]

Your hard lines and/or wheel cylinders could be contaminated. You might also consider switching to synthetic brake fluid; not only does it not hurt paint, it doesn't absorb moisture. My dad put a label on the 55 master cylinder that said "Synthetic Brake Fluid Only." The brakes were one of the few components that didn't need attention when I inherited the car; however, I've since converted to front discs.

[Rusty57]

Please don’t be offended by such a basic question. Are you sure that the discoloration is due to rust or is it the fine dark particles that wear off the seals?

My experience is that there is always a discoloration/darkening of brake fluid over time. In fact that color difference is what I watch when I flush brakes. The color change is obvious in the clear vinyl tube at the brake bleeder.

The probability is that despite the work done last year there is still rust/crud in the brake lines that was not completely removed by flushing. That’s almost impossible to do unless they are removed and media blasted or chemically treated. Of course if you go that far it makes more sense to just replace the lines.

[ward 57]

That's a good question. All new parts but the new parts could be sluffing off something in the seals. If it comes out black I'll know that's what it is. all I can see at this point is that it's not clear. Dang storm coming through again so it will be a few days before I can get to my detached garage.

[ward 57]

Your hard lines and/or wheel cylinders could be contaminated. You might also consider switching to synthetic brake fluid; not only does it not hurt paint, it doesn't absorb moisture. My dad put a label on the 55 master cylinder that said "Synthetic Brake Fluid Only." The brakes were one of the few components that didn't need attention when I inherited the car; however, I've since converted to front discs.

Synthetic and DOT3 do not mix. Only use synthetic on all new components from front to back unless completely voided of any DOT3.

[ward 57]

I just changed the fluid in my M/C. Blackish/brown color and no sign of rust in the M/C. Fresh fluid and will keep an eye on it.

[ICON 1956]

I suggest you run denature alcohol through the lines then fresh unopened brake fluid then bleed all of the lines. There could be a chemical reaction with a metal ( Aluminum) fitting that also could be the issue.

[ward 57]

The whole system was flushed last year when the rubber brake lines were replaced and the M/C and booster were rebuilt two year ago.

[Rusty57]

That color seems normal with age and use. That is why I flush any brake system at least once every 2 years.

[paul2748]

Have you checked the wheel cylinders. hard to believe that rust can travel back to the master but it may be possible.