The slave is essentially a smooth steel rod moving through rubber o-rings.
Unless the beast becomes water-contaminated and internally corroded (which can
happen and cause binding) there is very little way for the rod not to move when you're pressing on the pedal, even if the seals are bent and manky... (though worn seals will cause fluid seepage (internal or external) that will prevent the pedal coming back).
So changing the fluid can help in some
cases.. by flushing out any contaminants (water?) or air and "cleaning"/lubricating struggling parts. It's worth a punt.
Changing the slave is also worth a punt if bleeding doesn't work (the damaged/sticking parts may be beyond recovery).
The clutch on the other hand has more pressing (see what I did there?
) issues to contend with.
Assuming that the release arm pivots are not seized or corroding
... clutches get heavier because:
a) The fingers wear a groove where the release bearing presses on them. This makes the release bearing operate on them further away from the release arm pivot.
b) The clutch friction plate material itself gets worn away, which makes the whole friction plate move further away from the release arm pivot (the release bearing follows this movement.. so has to operate further away from the release arm pivot).
The further the release bearing is from the release arm pivot, the further away it moves from the arc that is described by the movement of the release arm (which is where you have the maximum leverage)... the more the pedal is pressed.
So essentially you are losing mechanical advantage the further the release bearing is away from the release arm pivot.
Clutch fingers don't
get lighter with age. Springs maintain their strength according to their effective length (Hooke's Law) and so unless all the clutch fingers becomes distorted, the clutch won't get weaker or stronger by itself. A clutch slips because the plate is worn out, not because the "spring" effect is too weak.
The groove in the clutch finger ends can wear all the way through and cause the release bearing to collapse into the hole it's worn for itself. Always replace the release bearing the same time as the clutch plate... and you may as well put a new Slave on there every second clutch or 100,000 miles depending how long your clutches last.
But it's usually