Replacing standard front brakes with 4pot Brembos
After bleeding I can get a hard lever but if I keep the pressure on after say 20 minutes the lever will ease further back to the throttle grip.  I have searched the old posts and have tried turning to left lock, tying the lever back overnight, hanging the calipers upside down with pistons pushed out as far as possible. I have been bleeding them only in the normal manner ie. from the bleed nipple at the caliper. I believe they can be bled in the other direction but not sure how to do it.
Open to suggestions
Don't know if this may be effecting things but the calipers are not seated perfectly central over the rotors- they are about 1mm off centre. The result being that the inside pair of pistons are pushing the pad a different distance to the outside ones. I can doctor them up to sit centrally to the rotor with a bit of work but if it is not going to make a difference I won't bother.
Please let me know what you think

A hard lever means there is no air in the system so that should be OK. having the lever slowly collapse would point to a leaking seal or a bad line that is slowly expanding with the pressure. Check for fluid under the rubber bellows over the master cylinder, if there is fluid there then the seal is bad because either the bore of the cylinder is corroded or scored or the seal itself is leaking. Check for fluid around the pistons at the caliper.
Hold the lever till it has collapsed to the grip and hold it there then check for distortions in the lines paying particular attention to areas around the fittings, the lines can have a leak in the inner which causes the bubble in the outer sheath but not to the outside.

Thanks Rick will give it a go

All lines etc ok. May have fixed the issue by forcing the pistons back into the calipers and then pumping them back up, did this a few times each looking for leaks etc. Must have forced some trapped air back the other way I think. Lever feels hard and appears to be holding firm and not collapsing. Promising at the moment anyway.

good to hear ...hope it works out mate