Well, this is the second second-hand brake I've bought that has had problems with the lever returning but the master cylinder return spring being unable to return the shuttle/seals assy.
This may also be a cause of some of those binding brakes out there.

The "O" ring on the shuttle has swollen and is binding in the bore of the master cylinder. And it undoubtedly would get a dousing if you were spraying a lube on the lever/pivots.

All the pivots are plastic bushed and in a lot of designs this means they should NOT be lubed.

I think I'll try the no lube or silicon lube when I overhaul my other 5s.

Any one know where I can get an "O" ring from without buying the whole Avid kit?

Cheers.

the local plumbers merchant near my has a tray of them.all sizes.
like 10p each.
so if you can find a good one near you.

Picked up a box of O rings of various metric sizes for a few quid at Aldi. This was a few weeks ago. Also worth trying Screwfix. Well worth having these items in stock

You are correct IMO on the original cause. Something I have been saying for a couple of years to friends who's Avid's I have had to rebuild.

We've probably got 100k 'O' rings at work.
They're not easy to measure, but if you can, & drop me an email, I'll see what we've got. The internal ID & the section dia are the key dims.
They're unlikely to be specials.
Failing that I've a few mfrs who can usually come up with "samples" off-the-shelf pretty quickly.

Not all o-rings will get on with dot fluid though. Don't ask me how I found that one out 😳

I was thinking that the "O" ring – 10mm OD 6mmID 2mm section – would not see that much(if any) brake fluid as it is behind the Hydraulic seal.
I have found some at simply bearings.co.uk and they are Nitrile but say nothing about DOT4 compatabillity. I'll pack between the seal and "O" ring with brake grease.

Ah, in that case ought to be fine, I was assuming it was on the wet side.

It will be in contact with fluid, as soon as the front seal passes the reservoir port the fluid then runs into the gap between the two seals.