Join Date: Mar 2016
Location: SF Bay Area, CA, USA
Car: 2006 WRX Wagon
I've run into similar. Bleeding typically is the solution.
Standard bleeding doesn't work though. Get yourself a C clamp (screw style) and then unbolt the clutch master cylinder from the transmission. Put the C clamp on the slave cylinder (use a small socket to create a 'flat' end on the rod end) so it cannot extend (just snug it on). Position it so the bleeder valve is facing UPWARDS to the sky, attach your clear bleeder hose, crank open the valve on the slave cylinder then do the usual pump/hold method.
The C clamp is used to not allow the rod to extend, thereby forcing all fluid to the evacuation valve. The valve needs to face upwards due to the design allowing air pockets to collect inside. With the valve upwards, gravity will do some of the work for you - allowing air to naturally go towards the evacuation valve.
Please be aware - do NOT use full force on the clutch pedal - use GENTLE hand/arm pressure. Because the C clamp is stopping the expansive motion, 100% of that force is going to the evacuation valve. I learned the hard way - I used full foot force and broke my C clamp (that sucked).
Once you are 100% certain that all the air pockets are gone, close the evacuation valve, remove the C clamp, bolt the clutch slave back onto the transmission (be sure to NOT go near the clutch pedal during this process) - and give it a test run.