Outer Limits 4x4 Board

The misses has a shorty GQ that started to develop a bit of a clutch problem. It seems that the clucth pedal has got a lot heavier whilst driving. When it first started i thought that it may have been the hydraulic fluid bubbling due to heat under the bonnet, and hot weather conditions. The fluid levels are spot on and it has a new slave cylinder. There is no slipping of the clutch whilst driving, infact it drives just the same as normal except, with an extremely heavy clutch pedal.
Any help appreciated....

if it is a boosted master it may have a vac leak or dud diafram.