Clutch Issue - Lossing pressure and gaining it back

I have a Toyota Corolla 2000 - 2.0D

From some time i have been noticing that my gears are not shifting swiftly mostly

Then suddenly one day my clutch lost its pressure for few minutes and i was not able to change gear i pressed clutch to its full limit multiple times but gears were in stuck condition.

After few minutes clutch gained its pressure back and gears started to shift but very roughly.

I took the car to mechanic , he showed me that my clutch oil was low and my clutch slave cylinder kit is leaking oil , i replaced the clutch slave cylinder kit and then added new clutch oil DOT3 grade as it was labeled on clutch oil reservoir , mechanic bleeded the clutch multiple times and i felt that pressure in clutch was back , on very next day clutch losses its pressure again for very few minutes , this time mechanic checked the master cylinder he said there was some cap that was lose and with in seconds clutch was working good he tighten it.

Now today again while in traffic after driving for like 30-40 minutes i felt that clutch lost its pressure very slightly but gained back pressure as i pumped it 2,3 times then it came back to normal.

I checked the clutch fluid line this time as i am sure that now both cylinders are ok , at some place i noticed that line has some oil marks on it , can leak in line can cause loss in pressure ? and how to be sure that clutch line fluid does not have air buble in it