silver flakes in brake fluid

[mweishaa]

Back in June I rebuilt my master cylinder with a kit from LMC. I developed a leak in the MC, so I took it apart today. What I found is a little puzzling. There was a lot of silver flakes in the fluid, reservoir, and in the MC.
You can see what I scraped out of the MC with a q-tip, and the pitting of the aluminum part. Not sure of that is what is causing it, or the 2nd pic below.

A better question is WHY is it happening. I'm running Dot3 fluid. Not sure what is going on here. Anyone seen this before or have any ideas?



Note the plating coming off.

[TR65]

My guess is that the brake push rod is not lined up with the MC. I had that problem with my hydraulic clutch MC. The push rod was not aligned when the piston was pushed in and it wore out the piston, eventually caused a leak.

Just a guess. (Why would it not be aligned?)

TR

[mweishaa]

Hm, I guess that's possible...but I don't see anything misalighned. I didn't see any wear marks on the piston. That pitting on it is what had me a little concerned.

I'm wondering if I should just switch to DOT5 fluid since it's not corrosive. Not sure if that's a good idea with drum brakes though... might have to do some research on it. I know on motorcycles you can switch over, and to be sure you can flush out the system with denatured alcohol. I know you can't convert the other direction, but 3/4 to 5 is ok.

[markeb01]

Do the flakes seem to be particles of the plating? If that’s the case perhaps sourcing an NOS domestic rebuild kit from ebay, or a name brand kit from a parts chain store might be a better option. Not everything made offshore is junk, but much of it doesn’t meet the same quality standards of products made here many years ago.

During the rebuild did you run a hone through the bore of the master cylinder casting? If it wasn’t trued up, new parts rubbing against the old bore may have dislodged some rust particles.

Another possibility being the master cylinder bore could be worn oversize after 45-50 years of use, or the piston could be undersize if it is a foreign production part. This could cause rocking during the stroke, scraping randomly and generating debris, and ultimately leakage. It’s been decades since I’ve rebuilt brake cylinders, so I’m not sure if the piston is original or part of the kit. And even if it’s original, it could still be undersize from decades of wear inside a cast iron bore.

Good luck figuring it out.

[mweishaa]

That was the piston that came with the kit... I don't remember if I still have the original around. It's possible there's disloged particles, but these are clearly silver. I never thought the bore would get that out of size, I figured the rubber parts were there to prevent them scraping. Kind of a mystery at this point.

[markeb01]

Originally Posted by mweishaa

That was the piston that came with the kit... I don't remember if I still have the original around. It's possible there's disloged particles, but these are clearly silver. I never thought the bore would get that out of size, I figured the rubber parts were there to prevent them scraping. Kind of a mystery at this point.

Some of the cylinders do actually wear out from age and use, and many suffered from being honed too aggressively during the rebuild process - accelerating wear in the bore. As far back as the mid 1970's some of my local parts stores stopped carrying the rebuilt cylinders and only offered factory new wheel and master cylinders. I asked why, and was told it was because too many of the cylinders that came in as return cores were just too worn out to be restored. The only fix was to have them rebored and sleeved which wasn’t cost effective.

The rubber seals are somewhat comparable to piston rings, their primary function being to prevent leakage. Just like in an engine cylinder, too much clearance between the piston and the bore can’t be corrected by the rings. I’m pretty sure in the older blue “Motors Manuals” they actually specified the clearance/tolerance dimensions between the piston and bore in wheel and master cylinders, as measured by a feeler gauge. They showed the entire process in the components rebuild section. Unfortunately I no longer have those vintage books to scan and share.

[TR65]

It looked like the the big piston diameters had wear marks on them (in the length direction) They wouldn't be machined like that. ??

The dot 3/4 fluid likes to absorb water, could cause the pitting?

I run dot 5 in my truck (with discs), but I would think it would be fine with drums.

TR