Odd clutch issue
 

I have a JDM 6 speed box fitted to my 2002WRX - Master Cylinder is std for the car, slave cylinder is the 6speed version that came with the box. I have an AP clutch (organic) and std flywheel, both 6 speed versions.

The issue I have is occasionally when accelerating hard and going up the gears fast the clutch has no pressure, e.g. I shift up to third then when I go to shift to 4th (very soon after on the JDM ratios) it's like the clutch hasn't come back up, the pedal just goes to the floor with no pressure. The pressure then comes back a second or so later.

Any ideas?

do you still have the pressure modulator valve fitted?? sounds most likely to be air or old fluid??

New fluid when the box was swapped, where would I find the pressure modulator valve.

it sounds like you have air in the system have you tried bleeding it mate.