P1 clutch loss
 

Random problem. My p1 seems to loose its clutch pedal over a run 30-60 minutes. Not always but now and again. Tonight it lost it very quickly.
The random thing is to fix it all I have to do is unscrew the reservoir cap and it's back to normal.
The car is only out now and again and has a version 7/8 sti top mount so the fluid reservoir is cut down.
Any suggestions?