Clutch master cylinder rubber?
 

Hi all,

ukmoods has pointed out to me that-

'You should never put a hole in the rubber seal, it flexes when the clutch pedal is pressed and acts as a barrier against moisture getting into the oil. The oil is hygroscopic so attracts water from the air, that is the reason for the little hole in the top of the cap, to allow the movement of air when the clutch pedal is pushed and released'

As I removed mine and cut the bottom off and then replaced the top half so it still seals around the cap, so the master then holds 5x more fluid.

Its just i have not come across this in any other cars master cylinder.

So before i go and get a new rubber, whats your view on this?

Moved to technical help forum

I'd agree with the advice given. Being above the brake pedal, the master cylinder is in a damp place as wet feet combined with a blower vent will cause moisture in the air around it.

That's my theory anyway.

Cheers, anyone else have views on this?

It's a system 'fit and forget' in factory.

Mike

In theory the 'sealed for life' system needs the rubber seal in place to act as a barrier to air and water ingress, and it allows the fluid level to drop without creating a vacuum in the reservoir. But in real life we all know that it's not a 'fit and forget' setup as they do need topping up now and again, and also need to be bled occasionally to flush out the bits of sludge that build up in the fluid too.

Although brake fluid is hygroscopic it's quite a slow process... what's the schedule for renewing the brake fluid, and how many people actually do it? But no news of people suffering brake failure due to water in the fluid (and it's much more likely there because of the higher temperatures).

So don't worry about the missing seal, the clutch is much more likely to fail due to poor quality materials, or gradual fluid loss rather than any moisture finding it's way into the fluid. And, if you just keep it topped up and bleed it through every 12 months or so, you'll be keeping the fluid pretty fresh anyway.

All motorbike hydraulic systems use the same system, but obviously they are in much wetter locations. None of them, are suggested to be sealed for life.

The idea is just a means to avoid the need to change the fluid, but really it is much better to change the fluid anyway. I suspect when originally fitted, that the capacity as fitted was increased by somehow pressurising the cylinder so as to scrunch up the condomand thus increasing the volume of fluid. Once the cap has been removed, the extra capacity is lost, so best to simply cut the condom away and just leave the seal part.

So far as moisture in the fluid is concerned, it is not nearly as great a concern as moisture would be in the brake hydraulics, with steel pipes and the possibility of the fluid boiling - the brake hydraulics are not similary sealed from air.

After learning a bit about this system, I reckon the rubber is to aide in preventing a vacuum forming in the reservoir and prevent the possibility of air being dawn in. Cutting it may just alter the feel of the pedal possibly??? as you may now have a slight vacuum resistance??? not sure it'll be a major issue as plenty seem to do it.

Our Rover 416 also has the condom cap , but located on the bulkhead in the engine bay, which is much better . In some 14 years it has never needed topping-up and never had a fluid change ; fluid is still clean . :}
If you are topping-up your reservoir , ask yourself where the fluid is going !
(My 75 is 11 years old and apart from being bled at my request when I had a new clutch and slave , has also never needed topping-up :} )

My clutch master doesnt have a condom it has a rubber bung type fitting. This must displace a large amount of fluid compared to the old system as the bung is about 4 cm tall??