That's great Hugo and all the above applies.
The Mazda box has an external slave so that's better than the MT-75 which has an internal concentric item. Is the knock you describe like the peddle suddenly springing up under your foot when you are releasing the clutch peddle. If so this would indicate a mechanical issue. so everything Dave has said is spot on --the way to go!
! is it sticking/jamming on the clevis pin at the foot peddle. notoriously soft and wear unbelievably!
2 check the links and actuating rods at the master cylinder and the slave cylinder.
If all is well there then it could be wear in the slave cylinder/master cylinder or just cruddy brake fluid and or possible seal issues on the piston, one seal can degrade and cause jamming!
3 next would be to actuate the clutch using the arm which goes into the gearbox and see if it is a problem there . this would indicate something closer to the gearbox.
First check the actual slave rod to clutch release arm function, they have been known to bend. Use your hand or a piece of wood to push the release arm on the gearbox throughout its range; if that's smooth and consistent then you're in luck as the gearbox can stay where it is! Fortunately the mazca box is fitted with the slave mounted externally'
Bleeding is a simple process here and you could try that as a first option, see if there is any difference as this will help diagnosis to the master cylinder.
You might be able to get a repair kit for either master or slave cylinder if its not to far gone and when you have identified what the fitment it.
