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a question about hydraulics.


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A simple hydraulic clutch system....single piston master cylinder, some pipework, a quick release coupling and the slave cylinder.

I wont bore you with the details, but I have had to change part of the pipework and remove the quick release connector. In place of the originals I have used copper brake pipe and a male and female brake pipe fitting. So far, so simple.

However, the brake pipe is smaller internal diameter than the original pipe. Does this matter?

The barb exiting the master cylinder is 3mm i/d. the original pipe was 4mm over a length of maybe 20 cms then the coupling which had 4mm i/d fittings. The copper pipe is 3mm i/d over the 20cm length. Surely for a stroke of the master cylinder piston, it will displace its volume of fluid no matter what size of pipe its going through?

The problem is the clutch is only partially disengaging. I dont know if its a problem with the pipe sizing or if there is still air in the system - its a nightmare to bleed it.

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It will only present a problem if you try and action the clutch quickly, there will be more resistance to the flow but negligable at slow speed, the faster the action the heavier the pedal will be.

Its probably a bleeding problem, try doing it slower as the same resistance might be compressing the air.

I had a mini with an impossibly stiff clutch except when actioned very slowly, the pipe was corroded internally.

I take it the slave cylinder has not moved and the bleed nipple is still above the inlet pipe at the highest point.

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The slave is one of these stupid concentric cylinders inside the bellhousing. It was a full two days work to drop out the complete power unit, seperate the box, change the clutch and refit last week. I fitted a new slave as a precaution.

It worked fine for a while, but gradually the gearchange became hard and eventually impossible. Bleeding it would solve it for another 20 miles or so.

I since found out that other forum users had trouble with this slave cylinder, made by Borg and Beck, where the quick release coupling would seep out fluid and let in air. This is the reason I removed it and fitted the copper pipe. The location of the master cylinder means that there is no way to get a pressure bleeder on it, so I am stuck pumping the pedal.

I have left the pedal wedged down for tonight - the sometimes helps tricky brake bleeds....no harm in trying, then I will have another go at it tomorrow.

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Yes, that is a good technique.

I thought afterwards that clutch pipes are usually a larger diameter than rake lines as the clutch displaces a lot more fluid, you may find that once you have got the air out it is heavy so you will have to practice slower gearchanges [:D]

I drove a Peugeot 207 (I think) diesel once, it was quite a hot one, a bit GTIish but they had this weird take on reducing emissions by limiting how fast you could hit the gas and they had fitted a viscous damper to the throttle pedal, it was un-noticeable when driving smoothly but when you drove it in anger it made your ankle ache, stupid really as they could have achieved the same, cheaper and without hurting the driver by the software of the fly by wire throttle.

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No Dave, as far as I understand hydraulics, the id of the pipe shouldn't make any difference.*  Think of the hydraulic line as a 'solid but flexible' rod connecting the two cylinders.  Move the piston in one cylinder and that movement is transmitted by our 'solid but flexible' rod to the other piston  How much the second piston moves is the result of  the ratio between this second piston and the first piston.  And that you haven't changed.

However (as I appreciate you know ...), if there is any air in the line, then the 'solid but flexible rod' is no longer solid.  Instead, the small amount of air in the line compresses and by so doing reduces the amount of movement transmitted to the second piston.  And now the clutch doesn't disengage properly ...

Sorry, but it sounds like you've got air in the line.  Just remember:

1.  Air rises.  Unfortuunately,  many people think they can get air out of the bottom of a system, often by using a bleed valve placed on the slave cylinder.  That may bleed the slave, but not the line.

2.  Air bubbles get trapped at joints, elbows, bends, junctions etc (including the cylinders themselves).  No amount of pumping will make a bubble move from these points - all you do is shift it up and down the line a bit.  Instead, try tapping the bend, flexing the hose etc, always with the notion that, once released, the bubble will rise up the line. 

3.  Pump slowly.  Pumping  fast simply breaks an air bubble into lots of little ones - which are very hard/impossible to remove until they've reformed into one big one.

HTH and good luck!


* E.g.  "A tube that is too small causes high fluid velocity, which has many detrimental effects.  In pressure lines, it causes high friction losses and turbulence, both resulting in high pressure drops and heat generation. High heat accelerates wear in moving parts and rapid aging of seals and hoses, all resulting in reduced component life. High heat generation also means wasted energy, and hence, low efficiency."


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Thanks all.

The old "wedge it down overnight" trick works its magic again.

I let the pedal up this morning and there was a gurgle from the master cylinder as air was released and it is working fine.

Just need to keep an eye on it for leaks now.

The pedal feels pretty normal to me. Its a complete new clutch anyway, so the pedal should be pretty light due to the replacement of the clutch spring mechanism - certainly no issues driving it this morning.

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