Bike was stored upside-down for a few weeks and the rear brake was pulling way back towards the bars. Got it upright and pumped it a little and noticed when the lever was squeezed, fluid came out from under the top cap. The brake was serviced by Hope a year ago so surprised if anything was worn out but bought a new diaphragm and put that in at the weekend. The flow is less but there's still fluid coming out. The lever isn't pulling to the bars though.
I know there's a hole in the diaphragm as part of the design so is it possible that I've just overfilled the reservoir and it's the extra that's coming out?

ta

you didnt pinch the new diaphragm or anything?

A set of Mono M4s that I had did that from new, in the end I fixed it by buying shimano XTs.

yep overfilled

There should be no hole in the diaphragm. there is a hole in the cap above the diaphragm at one edge If there is fluid above the diaphragm from overfilling it can come out when the bike is upside down. It will stop once the excess fluid above the diaphragm is gone

TJ – you're right – the hole is in the cap. I'm going to give it a couple of rides and hopefully it'll be ok…

Second TJ, Hole in Diaphram will make life difficult letting air in and spurting small amounts of fluid out on every squeeze (ahem).

If the lever was pulling back further than usual then you probably need to bleed the system.

Or, take the top cap and the rubber diaphragm off and top up the reservoir, then refit the diaphragm and cap. Zip tie the lever back to the bars and leave overnight. This usually allows any air in the hose to work its way up to the reservoir.