Adding Fork Oil
First, when I have the shock back together, spring out, I
extend it all the way, and use a tape measure to find out how far down the
spring seat is from the top of the tube. I then subtract a distance, usually,
and inch, and use that as a fluid level top measure. I then put the prescribed
amount of fluid in the shock, and cycle it, then, let it sit for a few minutes,
and go back, cycle a few more times, measure, correct the level if needed.
The main thing is, when the oil level is too low, and the shock has been cycled a few times in rapid succession, oil will not completely drain back into the damper. A slightly higher oil level, within reason, will ensure the damper remains under fluid, and not lose damping.
You might also do a modification from eons ago, tighten up the seal. Remove the seal spring for each seal. Look at it closely, and you will find a very small area, one "turn" of spring material that appears smaller than the rest. This is where the spring is put together to make it a full circle. Simply grab both sides of the depression, and twist the seal counter clock wise, to "unscrew" the two halves.
Once apart, one end will be flat, same size all the way to its end, the other will have a taper. I cut about 8 to 10 full turns of the spring off the FLAT end, NOT THE TAPERED END, then, dress the cut end to remove any burrs from the spring metal, and twist both ends back together, and put the spring back in the seal. This tightens the seal material to the tube, but does not cause adverse shock operation, and helps with sealing.
Another thing I will relate, do not run air in a front fork, all that does is to aerate the oil, and blow the seals in one fast hurry. If the shock is too light, pull the shock spring spacer, and make a new longer one out of either metal conduit, or PVC pipe.