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Old 11-26-2006, 05:43 PM
 

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Silicone for hydraulic clutch

Just finished bleeding the clutch using silicone fluid. But the throwout lever barely moves, and then only after 4 or 5 pumps. I've heard that silicone compresses or it may be that the master does not move enough fluid. Any other ideas/suggestions would be appreciated. I'm using the master from an austin mini that worked the clutch on the 4-banger, to work the clutch on a Porsche transaxle.

Thanks,
Animal

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Old 11-26-2006, 05:53 PM
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silicone fluid

Silicone fluid does not compress. You have air in the system or bad/defective cups in the m/cyl. or s/cyl. My 1st guess.

I am not sure of the fluid displacement of the m/cyl. and s/cyl.. to evaluate if it is compatible with each other
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Old 11-26-2006, 06:01 PM
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Heres some instructions on bleeding a hydraulic clutch
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Old 11-26-2006, 06:10 PM
 

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Silicone for hydraulic clutch

Thanks for the bleed technique. I'll definitely try it.
Animal.

As for the parts, they are all brand new.
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Old 11-27-2006, 07:49 PM
 

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Silicone for hydraulic clutch

The instant I bled the slave cylinder, the pedal went hard! Real hard. But it would not move the the throwout lever enough to allow a shift. That's the next problem to solve. Is it possible that the Mini master cylinder is too small to overcome the resistance of the Porsche racing pressure plate? Any other ideas????????????
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Old 11-27-2006, 07:58 PM
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No, it depends on how much fluid it moves, a small
cylinder working a large cylinder increases the pressure
applied, so you need to know if there is enough volume
coming out of the master to adequately move the
piston in the slave...

K
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Old 11-28-2006, 02:22 AM
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I put silicone in my girls 90 jeep Cherokee because the tran guy said they had a tendency to go bad in short order and thought the silicone would be less destructive, it worked fine BUT we first found the hyd line bad then finally the the m/c for the hyd clutch. finally after every thing was changed it has been working fine for over a year. Not sold on this for all jobs toooo expensive but for this I would do it again. Ed ke6bnl
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Old 11-28-2006, 07:27 AM
 

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Silicone for hydraulic clutch

Thanks for the insights, guys. I will try to find out more about the relative piston diameters. The very hard pedal does appear to mean that the m/c piston is smaller than the Porsche slave.
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Old 11-28-2006, 09:08 AM
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Quote:
Originally Posted by Animal
Thanks for the insights, guys. I will try to find out more about the relative piston diameters. The very hard pedal does appear to mean that the m/c piston is smaller than the Porsche slave.
I would say the other way - the m/c piston is much larger than the Porsche slave, meaning you have not enough hydraulic pressure to move the stout pressure plate. Mathematically, Larger bore (larger piston area), same applied force, = less pressure

If the clutch pedal was going to the floor and not moving the throwout bearing, then the guess would be that the m/c diameter was much smaller than the porsche, hence not moving enough fluid volume.
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Old 11-28-2006, 04:04 PM
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Silicone for hydraulic clutch

Force divided by area = pressure. So a constant 100 LB force [your foot] applying that force to a 2 inch piston would equal 50 psi of pressure.

That same 100 LB force applied to a 1 inch piston would equal 100 psi of pressure.

I think hotrodf1 has you looking in the right direction.
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Old 11-29-2006, 02:46 PM
 

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Silicone for hydraulic clutch

So what seems to be agreed is that the m/c diameter has to be bigger than the slave cylinder diameter in order to push as much fluid as possible which will in turn move the slave cylinder as far as possible. I am looking into the relative diameters. Update at 11

Thanks all.
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Old 11-29-2006, 02:53 PM
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Look at it this way,

The bigger the MC bore, the further the slave moves, but the harder it will get to push. Even to the point you can't do it with your leg at all.

The above ****umes the slave stay's the same size.
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Old 11-29-2006, 03:02 PM
 

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Silicone for hydraulic clutch

Any rule of thumb on relative sizes? Like 4 to 3 or 5 to 3 M/c to slave?
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Old 11-29-2006, 07:16 PM
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You need to know the amount of travel the slave requires
to move the PP off the clutch disc(DP), then you need the
area of the slave, if you can post those two items, I can
figure out the amount of fluid you need to move the slave
that distance, and how much travel the master will need
for a given bore/cc displacement... And still let you push
the clutch pedal without breaking your seat back...

K
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