I converted my brother's 98' prelude to manual a few weeks ago. The problem I am having is when the car has been sitting for 1hr or more the clutch feels perfect. After about 20mins of driving the clutch engages at the very top as I let go of the clutch pedal. It almost feels like it starts to slip a little. It is a brand new clutch. I bought all of the clutch lines, clutch mc and slave cylinder from honda brand new. There is no leaks that I can see. What could cause the engagement on the pedal to change?

 

make sure the nut on the master cyl to pedal rod is tight, otherwise it could move. Also make sure the slave cyl is bolted down tight.

Otherwise you have a leak somewhere.