Clutch fluid disappearing... Infrequently?

[phatman5000]

I have a strange problem with my 350i. About 6 months ago, as I was half way to my destination, the clutch pedal went very floppy. By the time I got there, changing gear was almost impossible. I checked the master cylinder reservoir, and it was empty. Fine.

Filled it back up, and the clutch came back. Fine.

It then stayed full over 5 months of regular driving and didn't go down. So I assumed maybe the level hadn't been checked in years and it had just naturally needed topping up.

I then went to drive away while it was parked in gear, and no clutch - fluid gone. Topped it up, clutch came back and held its fluid fine. Then parked it in gear again a few days later, and came back to a floppy clutch. Same thing this morning.

So... What causes a clutch to suddenly lose a reservoir of fluid, when it holds fluid fine most of the time? Am I boiling it? Or is it somehow related to parking in gear?

I'm very confused.

Thank you!

[KKson]

I'd guess just old seals. Some days it holds, and then other days depending upon how the seal sits when you park up, it weeps out. I'd pull the slave cylinder off and have a look at the condition. Might be there's some rust that's being picked up by the seal. Replacements are cheap. On my SEAC I went for a slightly larger bore which reduces pedal pressure. Motoclan stock them.

[BlueWedgy]

I Concur, had the same issue, replaced the rubbers initially, went bad a couple of months later so ended up buying one off fleabay, been all good since.
This is the one......

https://www.ebay.co.uk/itm/324193358334?epid=60170...

[LLantrisant]

more or less the hydraulic-system is a closed system, (besides the small sniffing hole in the resevoir-cap).

the fluid cannot evaporite and its boiling-temperatue lays far over 200degees celsius.

so, if fluid gets lost it got lost through leaking seals...time for a new master & slave or a seal kit.
just an advice: do not wonder with new master&slave if you may need to replace one or the other within 1 or 2 years again...the nowadays quality of those parts is often a nightmare.

therefore some say: if the bores are still ok, try to save your original parts with an original (NOS) seal kit

[BIG DUNC]

I am about to change my Land Rover clutch cylinders, which are of a similar vintage. Not sure whether they are the same as a wedge or not. The words of wisdom from the Land Rover man I use was, at that age, to do both the Master and Slave cylinders at the same time. If the seals are degrading they contaminate the fluid, so the likelihood is, you change one cylinder then a few months later you have to do the other.

[jev]

When I had this on my Triumph Spitfire, it was the slave cylinder rubbers. Went suddenly and about every three years.

My theory was that it was the age of the cylinder that led to the repetitive failure. But it was cheaper + easier just to do the rubbers as and when needed.

[adam quantrill]

I've swapped over to LR master and slave - they have different threads so I added adapters.
Also salvaged the pushrod off the old slave. They work just the same and are only about £12 each.

Yes it's probably draining out of the slave when it goes dry so replace that 1st.

[Wedg1e]

I've had clutch fluid loss from the master cylinder, it runs down the firewall, under the carpet.
It's allegedly not helped by the arc through which the clutch pushrod travels (it's been mentioned on here before, feels just like the old days ) which causes a bit of sideways thrust on the m/cyl piston.