Gary Z

It is commonly claimed that heat is the reason our clutch fluid turns black. I think that the true cause is contamination by the black grease (probably molybdenum disulfide) that lubricates the slave piston. This grease is isolated from the clutch hydraulic fluid only by a rubber seal that moves with the piston. Disassembly and examination of an old slave has convinced me that some of the black grease will inevitably find its way under the seal and into the working fluid. Brake fluid gets hotter than clutch fluid without turning black.

wrencher

iTrader: (2)

Brake fluid is malso hydroscopic. It will absorb moisture & darken for that reason as well.

02silvaZ

iTrader: (12)

good info

02silvaZ

iTrader: (12)

my slave just bit the big one, changing it out tomorrow

Gary Z

Yes, darken. But my clutch fluid has been very black, blacker than my brake fluid has ever been.

Honda Nick

iTrader: (1)

This is a topic of much discussion on CorvetteForum, the accepted reason is that the black rubber itself is dissolved by the DOT3 fluid.

Gary Z

This theory is certainly incorrect. The rubber seal in the slave I disassembled is in perfect condition after 25000 miles. Very small numbers are still visible on the seal. Brakes have similar rubber (neoprene) parts exposed to DOT3 fluid.

Honda Nick

iTrader: (1)

Believe what you want:

Gary Z

Thanks for the pictures. The damage in the last picture appears to me to have been caused by abrasion. It is unreasonable to conclude that the seal has "dissolved" or in any way been attacked by DOT3 fluid. You can soak the seal in DOT3 fluid for many years - it will not dissolve.

BlackScreaminMachine

iTrader: (17)

This is my observation.

I suck fluid out of the resivor(sp) and its pretty dark. I clean, add new fluid and aggitate the clutch 30 times fast. then I change and get it pretty clean.

I noticed one time when cleaning up the black stuff came out of the line. I beleive it was all in the slave and it was either heat killing the fluid down there and what I was doing isnt fixing it. Its gotta be flushed from the slave

Gary Z

I think the procedure you describe is very good maintenance but if your fluid is quickly getting dark it probably indicates that the seal is beginning to fail. You are correct that cleaning isn't fixing it. You won’t be able to flush the “black stuff” from the slave.

BlackScreaminMachine

iTrader: (17)

If I run fluild from the top to the bottom just to ensure its all fresh is my 1st and cheapest step, I got a Original clutch with almost 37k, im sure I am due for a upgrade but do the cheapest and easiet step first, im doing that to day, ill post the findings.

AllThrottle4U

iTrader: (14)

I always thought the black was from the clutch material dust being "raked" into the fluid when the seal travels over the cylinder.

BlackScreaminMachine

iTrader: (17)

The fluid system is a closed system, its pressurized, there is no way clutch meterial can get into the fluid system.

Gary Z

I considered this possibility simply because the clutch dust is, like the grease, very black and it does appear that dust might find its way under the slave’s outer sleeve. But I carefully inspected the slave’s grease and couldn’t detect any gritty feel. Since the grease is always in direct contact with the seal, I think the grease is the most likely source of the contamination. Someone has undoubtedly answered the question by chemical analysis.

AllThrottle4U

iTrader: (14)

Interesting. I invested in a remote clutch bleeder line when I changed to the ls7 clutch. All I can say is it's the best investment made to my tranny system. Bleeding is a cake, the hardest part is jacking up the car.

ibanez7

iTrader: (7)

The hydraulics in our cars is a closed system. So i doubt the blackness is from clutch dust. But i could be wrong.

I feel the reason for the fluid turning black is a combination of things.

First the rubber hose from the reservoir to the master, along with any other seals throughout. DOT3 or 4 fluid synthetic or not, may not completely eat at the rubber component to the point of destroying them, but I am positive it does eat at them enough to pollute the fluid.

Heat is definitely a factor too. Many people do not feel that hydraulic fluid for manual cars is exposed to high heat but it is. Besides the heat build up from the clutch engaging and disengaging your clutch hydraulic fluid is exposed to heat from the engine and drivers side header. Also think about this our engine blocks are aluminum, bellhousing aluminum, transmission case, aluminum. All three of those act as one giant heat sink. So with the Slave mounted around the input shaft and fastened to the transmission, now the slave becomes a heat sink too. Unlike brakes the entire hydraulics system barely sees fresh air. At least brake rotors have air vents and wheels to help cool off the brakes. As anyone notice how hot the shifter stick gets after driving or abusive driving. Heat causes the fluid to boil and the elements that make up the fluid then begin to separate and absorb moisture.

My reason for thinking this is because of the fluids I have used:

GM hydraulic brand - black and thick within 5000 miles and caused shifting issues.

Valvaline synthetic - black within 20000 miles, no problems shifting

Wildwood high temp race fluid - clear but with black chunks within 5000 could not shift. Fluid ate the seals, slave and master where junk.

The maximum boiling point of the fluid is very important. To little and the fluid goes to crap, too high and fluid eats everything.

MeentSS02

iTrader: (11)

I'll have to see how Castrol SRF does since it has one of the highest boiling points out there...the ATE Super Blue that I replaced actually didn't look all that bad.

Mike454SS

iTrader: (4)

How long did you run ATE Super Blue in there? Any problems associated with it? I run that in the brakes on all my vehicles, and I'd like to flush the clutch fluid this weekend as well...I wanted to try to be sure it's ok in the clutch hydraulics and if not I'll probably buy Valvoline Synthetic or Castrol SRF. I just figure it'd be easier to run the same fluid in both if I can.

GR33N GoblinM6

iTrader: (14)

i also use ATE Super blue in my clutch and in my brakes.. had it in there 5000 miles and still looks great at the top