Dammit’s adventures in mountain biking

1.The fluid has leaked away and there's practically nothing in there

This.

I would add, if it's leaked out I can't see where it's done it, and it seems to have done it between yesterday (worked perfectly bar rubbing) and today.

Perfectly possible of course that I've simply missed where the fluid has emerged.

Inner pistons stuck in their bores

This I reckon, although piston maybe sticky rather than stuck, explaining why it worked before taking it off the car...

'If I pumped the lever maybe 20 times I could get a little bite, but that vanished as soon as I stopped working the lever.'

This would normally suggest air in the system, no?

This would normally suggest air in the system, no?

It was extremely marginal, previously when I've had air in there you could (3-5 pumps) get the lever feeling fine. In this case the difference was between the lever going to the bars and nothing, and the lever going to the bars and there being very slight grabby braking.

A combination of sticky pistons and bubbles in the system I’m going for

I had customers who experience these effect to find the system was leaking, moreso a leaky system mean not all the mineral oil is spread to the piston.

Can be wrong naturally.

Leaks could be from:

• Calliper bleed port
  
• Calliper/hose junction
  
• Around the pistons (it's not this, totally dry)
  
• Lever/hose junction
  
• Lever bleed port
  
• Somewhere else?
  

I think its possible that when you first pushed the pistons back it has blown a seal somewhere. I'm not sure where though but I'd guess its a the lever somewhere.

The way you describe pumping the lever 20 odd times would make me think there's been a loss of fluid somehow.

Probably (3). Hoping it's (4).

I think its possible that when you first pushed the pistons back it has blown a seal somewhere. I'm not sure where though but I'd guess its a the lever somewhere.

This strikes me as plausible. I need to play "spot the Royal Blood" tomorrow.

I've done this once before and blew the reservoir bladder/seal thingy at the lever.

I admire your perseverance with these brakes.

These brakes are, and I don't say this lightly, going to fucking work. I didn't think it was going to take quite so much buggering about to get to that point, but here we are.

Sounds like you might benefit from one of these -

https://www.ebay.co.uk/itm/Piston-Relase-Tool-For-Magura-MT-4-Pot-Calipers-/264532855159?_trksid=p2349624.m46890.l49292

I've got one, they're as useful as they are expensive - very. Really makes servicing 4 pot brakes a doddle.

Ordered, thanks.

Good article...
https://nsmb.com/articles/clean-your-brake-pistons/

Sounds like the pistons need some manual coaxing to move freely rather than a clean but the same methodology applies. You can do it with a syringe in the lever bleed port to relieve excess pressure off internal seals as you’re pushing pistons back as @TotalShanner says (just make sure it’s topped off with fluid to avoid sucking air)

I saw two nicely mucky MTBs on the top of your car by Horniman on Saturday. Brakes looked fine then...

The brakes worked then! Also, I think my car's full name is "the filthiest car in South London".

You haven't seen mine....

Ok, place your bets (1 through 4), as I have the bike in the stand.

Definitely 4.

Ok, piston spacers in, pull the lever - clamps the spacers in place, but again the bite point is with the lever against the bars.

Piston spacer between a pair of pistons, tyre lever holding the external piston of the free pair in, the inner piston extends when I pull the lever. The piston stays extended.

Same if I swap the spacer round.

So! The inner pistons do move if I isolate them.

Looking like Goblins to me so far.

Zero signs of leaking fluid anywhere, so far.

I’m sticking to 1.