The Reason Your Clutch Fluid Turns Black
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The Reason Your Clutch Fluid Turns Black
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.
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Originally Posted by wrencher
Brake fluid is malso hydroscopic. It will absorb moisture & darken for that reason as well.
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Originally Posted by Honda Nick
This is a topic of much discussion on CorvetteForum, the accepted reason is that the black rubber itself is dissolved by the DOT3 fluid.
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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.
Last edited by Gary Z; 08-10-2007 at 10:50 AM.
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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
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
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Originally Posted by BlackScreaminMachine
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
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Originally Posted by AllThrottle4U
I always thought the black was from the clutch material dust being "raked" into the fluid when the seal travels over the cylinder.
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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.
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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.
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.
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Originally Posted by MeentSS02
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.