Firstly, I know I’m an idiot. Tried to bleed my Mono M4 yesterday and forgot to put the brake block between the pistons. 🙄 Cue one of the large pistons pushing out, and fluid ending up everywhere. I’ve tried pushing the piston back to its original position, but presumably I’ve bust a seal or something, because I can’t get any pressure back into the system; there’s just no resistance at the lever.

So, could someone explain in word of three syllables or less what I’m likely to have done/damaged, and how I can go about rectifying it please?

With the piston popping out you’ll have introduced air into the system hence the soft lever feel. Best bet is to start over again bleeding. When this has happened to me I had to try lots of things to changing the orientation of the caliper to encourage the air towards the bleed nipple, tapping gently the caliper to dislodge any air bubbles.

Worst case scenario is that you’ve damaged the piston seal when pushing back in if it went back in at an angle in which case its piston out, using a pick remove the old seal insert a new seal. Wipe with a bit of dot fluid or rubber grease push piston back in and bleed the system again.

As above.

If there is air. Then caliper above reservoir. Bleed as normal. With bleed nipped at highest point of the system. Once you’ve done that. I’d carefully redo it in normal position.

All sorted thanks. I thought the complete lack of any ‘feel’ in the lever meant I’d done something awful, but took advice and restarted from scratch. Looks good now. 🙂