1978 Other Toyota Models Build up of fluid pressure of clutc

Own a Toyota Hi ace van. Just recently replaced the clutch wiih new cltuch package. Eg. Plate. Spigot bearing and Throw out bearing. I have enough free play when firts off while everything is cold. Once driven for about an hour I find there is no free play left. If driven any furhter I find that if I apply pressure to the pedal slighly the clutch will slip. This has my mechanic baffled.
Any help of this problem would be greatly appreciated.

Ps. Fly wheel was also machined at the time of replacing with new parts