Thanks everyone. I'll give it a try in daylight tomorrow.

I rather think that we are at cross purposes using the word "bayonet". To me it's those two pins on the side of a light-bulb.

maybe better visible here, no bayonet, just a rim that clicks in the rubber seals.

From my experience, they tend to leak when cold, even with quite new seals. Never bothered to worry about, only check the amount of LHM more often on winters...

And another thing, they seems to leak more when not driving...

Thanks very much everyone. I pulled it out, carefully but firmly, if that makes sense.

The seals look OK but the reservoir has a crack, which was in a place where I couldn't see it until I took it off and started cleaning away the dirt.

I've ordered a new reservoir and new seals.

And no, that's not what I'd call a bayonet fitting (pins and a twist). It just pushes on .... and off. But then, actual bayonets stopped having bayonet fittings a long time ago.

No pins on these bayonet fittings, nor on such fittings so described by many other manufacturers...

https://www.sipuk.co.uk/connectors-coup ... ptors.html

Other suppliers seem to have different names for those types of air line connectors, Ken:

https://uk.rs-online.com/web/c/pneumatics-hydraulics-power-transmission/pneumatic-adaptors-fittings-couplings/pneumatic-quick-connect-couplings/

And then, of course, Wikipedia speaks (twice):

https://en.m.wikipedia.org/wiki/Bayonet_mount

https://en.wikipedia.org/wiki/Bayonet_mount

'Bad' becomes good, 'wicked' becomes cool, 'cool' becomes great, now bayonet becomes push fit, and twist becomes tilt. Always been a push and twist action to me using pins into right angled slots or grooves.

Like fast cars - some things I don't want to keep up with.

Got the grumps this-morning.

I have had the same problem and agree with the comments. Leaks more in the cold, also when not used for a few days (especially in the cold weather). I've replaced the seals but the level is still going down slowly. Didn't notice any cracks in the reservoir. I'm just wondering if I've got the reservoir fully home. Does it need quite a lot of pressure to get the rims of the reservoir to engage in the grooves in the seals?
Don

Don,
having carried out removal and refitting of master cylinder reservoirs on several hundred occasions during the last 35 years, this is what's worked for me.
( Click on the link for full text.)
In addition to using LHM as a lubricant, a light coating of grease on the seals and the reservoir spigots also helps to ensure an easy and leak-free installation...
.
Dual master cylinder, bayonet fitting. by slcchassis, on Flickr

Judging from the photo, it's not 'Grim up North' at all.