Hi guys

After driving on a highway for 40 mins at 90kmh the clutch pedal went straight to the floor, had zero feeling and I was unable to change gears. I checked the reservoir which was full however there was little to no movement on the slave cylinder. I assumed the fluid had boiled and sat the car for 20 mins. Feeling returned to the pedal with the engine off, but as soon as started it become quite soft again, which I assume was due to the power assist. Pushing it all the way to the floor I was able to select a gear and drive home.
Today the pedal is as soft as yesterday. I will replace the slave cylinder, rubber line, flush and bleed. Apart from this what else could lead to the above symptoms? Also, how do you check the clutch in a GB TB42; I'm told they don't show signs of slipping and just let go.

FYI - the car is off the road while I also try to resolve large coolant leaks.