Clutch Hydraulics

DiscoII Td5 When I drove the car for any distrance, say 200km at 110km/hr (using cruise usually) the clutch would lose a lot of pedal. After advice from this NG about fluid leaking past the pistons, both master and slave were replaced with new. However, the problem still occurs although to a lesser extent. As it really only occurs when driven continuously for 200-300km at a constant speed, could heat build-up have anything to do with the problem? Steve W (in Aus)