Next time you drive the car, put it in 5th gear and go about 20mph. Leave it in 5th and put your foot to the floor at 20mph. If the mph gradually come up like it should then it's more than likely ok, if the RPM's start to climb quickly or jump, then your clutch is probably slipping....that is a quick and easy test....
Have you tried bleeding the slave? And where is the engagement of the clutch? At the very bottom of the pedal travel right before it hits the floor, or in the middle or somewhere at the top?