'Condom' in clutch master reservoir ??

[Sonic ZS]

Does anyone know the actual purpose of the 'condom' that drops into the top of the clutch master cylinder ?

I've recently been adding a remote reservoir to a manual diesel, using a Panda/500/Ka reservoir, and it had exactly the same black rubber thing inside, the moulding was identical to the one in the 75, so not model specific.

Initially I thought it may be there to reduce the quantity of fluid required at the factory, but I'd say it costs more to fit the condom than the equivalent amount of fluid.

Anyone know why it's in there....???

Thank you in advance

[Rogue]

At a guess it operates both as a seal and a diaphram to allow fluid levels to change in the resevoir but still stop moisture getting to the fluid from the vented cap.

[Les4048]

Perhaps this will explain it

https://mechanics.stackexchange.com/...utch-reservoir

[Mike Noc]

Originally Posted by Rogue

At a guess it operates both as a seal and a diaphram to allow fluid levels to change in the resevoir but still stop moisture getting to the fluid from the vented cap.

Spot on - when you press the clutch pedal down the level in the reservoir rises. To allow for this, and expansion due to heat, whilst also keeping the hydraulic system air tight, the rubber diaphragm is fitted.

As the clutch plate wears down, the fluid level in the resevoir also rises, and the diaphragm allows for this.

If you look at the clutch reservoir cap it has a small breather hole in the top. If you leave the diaphragm out and fill the reservoir to the top then when you press the clutch pedal down fluid will spray out through the breather until the excess is expelled.














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[Sonic ZS]

But surely when you depress the clutch pedal, you force fluid down the pipe to the clutch, not back up into the reservoir Mike ??

So am I right in thinking that the fitment of a condom allows manufacturers to say the clutch hydraulics are 'sealed for life' and hence reduce their published service times ?

[Mike Noc]

You'd think so but it doesn't work like that.

The level rises in the reservoir all the time you are pushing the pedal down. As the pedal starts to go down the supply port is open, so fluid is pushed back up to the reservoir by the piston. However once that port is shut off the level continues to rise, so maybe there is a secondary seal on the piston shaft - I'll have a look next time I have one apart.


You can see the effect with the GhettoTech easy fill and level indicator fitted:






Yes the condom ensures the system is sealed for life, unlike the brake reservoir which is open to air and where the fluid is changed at the recommended service intervals, or should be.










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[Sonic ZS]

Thanks Mike, you are right I've just asked a friend to press the clutch pedal down whilst I watched the level in the new, engine bay mounted, reservoir.....and it does indeed, rise slightly !

Will be keeping a regular eye on this now, just to know what's going on in a previously hidden hydraulic system

I should add that both my condoms have been modified, so that only the upper seal of both remains, and the fluid will now get changed on a regular basis.