97 4runner 5spd 4x4

Some how air is getting into my hydraulic system but im not losing fluid. This happened the other day while driving to work, previously it was working fine

Ill bleed the system and have a nice firm pedal, I will then go for a test drive get a few good shifts then the pedal loses pressure.
I have swapped out master and slave cylinders and have the same problem.

other stuff I have done:
Removed the torsion spring from clutch pedal a couple years ago.
sprayed some high temp grease on clutch fork pivot to stop it from sqeeking
I don't have a clutch pedal return spring on, its been like that for a year, should I put one on?


what should I do next?

 

In all of your swapping around you have a poor connection some place .

allowing air to be pulled into the system.

It does not take much just a little bit of dirt will cause this to happen.

It could even be the bleeder valve that is causing the problem