Run the car till brakes are binding then take off wheels , if you can't push back Calipers as normal then open a bleed nipple and try it , if they then go back problem lies with flexi pipe
It will have an internal blister acting as a one way valve for the fluid
I'm assuming master cylinder is same type and there was clearance when you tightened it onto the servo ?
John
Tried that john and the Pistons pushed back in ok, I'm going to try the adjusting screw on the rod in the servo, it's my last hope !