My hydrolic clutch has just been repaired all new from master cylender to plate but will not stay bled for more then bout 50 miles what could the problem be

Can you see leaking hydraulic fluid anywhere? The principle is very simple The master cylinder behaves like a syringe. You press the clutch pedal and fluid is forced by the master cylinder plunger to the slave cylinder mounted on the side of the gear box. The slave cylinder is like a syringe in reverse so that the fluid pushed into it from the master cylinder causes the plunger to extend into the clutch/flywheel bell housing and engage with the end of the clutch bearing release lever. The slave cylinder has a bleed screw on the top of it to release any air that has got into the system. Ensure that reservoir is full and ensure that a sheet of polythene has not been left under the screwed on cap: sometimes done to prevent excessive fluid loss whilst working on hydraulic systems. Make sure that the hydraulic unions at the master and slave cylinders are nice and tight. make sure all bled points are similarly tight. The only possible remaining places for air ingress (and by default fluid egress) are the cylinder fluid seals on the pistons... get under and look for leaks when you can get a colleague to pump the clutch repeatedly