Bled my rear Mono M4 last week as the lever was pulling back to the bar before anything much happened – felt great after bleeding, then did a 2 hour blast round Thetford – brakes still felt fine, went to move the bike tonight and the lever pulls back to the bar again.
I presume I need to replace some seals? is it most likely to be the calliper end?

Probably caliper end but I would bleed it again first to be sure seeing as bleeding them is so simple

Have you put the reserviour rubber thing in the right way round ? Has it seated properly ? Its easy to cock that bit up.

Has the bike been stored upside down by any chance?If there's air left in the reservoir, then it can get into the pressurised side of the system and cause this problem.

It's the 2nd time I've bled them – I assumed when they went soft after the first time I'd just cocked up the bleed somehow so did them again on the Monday and they were fine 24hr later when I went to ride.

@Trimix I think my reservoir rubbers OK but I'll check it.

check the diaphram's not split somewhere.

Has the bike been stored upside down by any chance

No it's been upright all the time – even when I was riding it 😉

Sounds like it could be master cylinder seals then – but I am not sure.

everything looks ok but pads are a bit low and there wasn't a great deal of fluid left in the reservoir.
I may well rebleed, replace the pads and see what happens.
(or get them serviced at 18bikes for 45 quid inc pads… which is tempting as the price for pads+seals+pistons must be close to that!)

Make sure you roll the diaphragm on when you replace it, displacing fluid as you go along. This ensures air is not trapped underneath it. There's a short video on the hope website demonstrating the proper technique. Sorry if you already Know this.

@drew – ah cheers, no I didn't know that…

Right brake bleed#3 coming up.

yes that sorted it, new pads and rolling the diaphragm on, brakes have been fine since.