If you have a coaxial slave cylinder then most likely its filling the
bottom of the bell housing with fluid. Pull the rubber access plug on
the side of the bell housing and check with a mirror and flashlight. If
nothing there then check inside the truck around the clutch pedal rod or
carpet.

You say, "tiny bit lower," but is a tiny bit a teaspoon, tablespoon, few
ounces?

Unless the level is below MIN on the reservoir, I see no reason to top it
off -- given the symptoms you are having. (There are lots of reasons to top
it off, but the symptom set you are having does not prompt me to fill the
reservior.)

Your clutch is activated hydraulically, exactly the same as the brakes --  
with a minor variance. The variance is, as you rest yoru foot on the brake
pedal to hold your truck inm place, you apply pressure to the sustem. The
clutch has massive springs inside that apply pressure to the system. The end
result is the same, and I will describe it for you from the perspective of
the brakes.

As the pressure on the pedal is maintained, the fluid inside the Master
Cylinder is held up on two different fluid circuits. There is a piston
passing between two different chambers in the MC, and o-rings attempting to
keep the fluid separated -- the chambers are designed to regulate braking
pressure on two sets of wheels, so if one set looses brake pressure, hopes
are the other set will retain functionality long enough to stop. In any
case, a faliure of the o-rings allows the piston to move because fluid on
one side of the o-ring can pass to the other side, and the affect you feel
is your foot sinking as you wait for the light to turn green. As your foot
falls and brake pressure drops, the vehicle may begin to creep; you remedy
this by pumping the brake pedal a few times to bring the pressure back up,
which raises the pedal off the floor and you continue waiting for the light
to change, and your foot sinks again.

On the clutch, the same forces are at work, except the pressure is applied
by the massive springs in the clutch itself. The symptom set is a bit
different because the pressure comes from a different place, but at the end
of the day, you still need a new Clutch Master Cylinder. There is a piston
connected to the clutch pedal, and it has an o-ring that seals it to the
bore inside the MC. You could simply press the clutch pedal several times
while the trans is in N, this will build hydraulic pressure in the clutch
system, allowing eventual selection of the desired gear. You probably have
difficulty selecting any gear, and more difficulty on the first or second
gear selection after not selecting any new gear for a long time. You notice
that 1st and R are most difficult to select because the gears moving due to
the engine and the gears not moving due to the driveline are the most
difficult to engage -- as you make a shift after the turck is already
moving, the selection is less difficult because the parts are already
moving, and if you make the gear change at the right moment the gearsw will
be moving at the same speed and the change can be done without using the
clutch at all.

BOTTOM LINE
The Clutch Mater Cylinder has an internal leak -- failing o-ring -- that
makes the pedal action ineffective until you pump the pedal several times.
The fact that you have filled the reservior is coincidence.

If you sat at a Red Light in 1st gear and the clutch depressed the whole
time, you would notice that the clutch engages at different points, in
extreme cases the truck might creep on its own, but you probably uswe the
brake at the same time and do not notice. Normally, thbe clutch should start
to grab at about the half-way point, but you should notice it grab nearly at
the floor on some occasions, and further up on others. If this happens, the
clutch springs have caused the fluid to leak past the o-ring(s) and the
clutch plate is beginning to approach the flywheel all by itself.

In both systems -- clutch and brake -- the fluid pushes on slave cylinders
that actually activate the mechanism at the other end of the pipe. These
slave cylinders can leak and give the same symptoms, except they will leak
to the outside and you will see fluid coming out. You will also see a
noticable drop in the fluid level in the respective reservior -- the fluid
will drop from MAX to MIN or below, and this sort of drop is never
characterized as "tiny bit lower."

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