The Lee Safety Disc measure makes a wonderful, inexpensive pistol powder measure, but can be temperamental with leakage of some of the fine grained powders.
With a little "tweaking" it is becoming my favorite measure for pistol and small capacity rifle cases.
First off, I utilized the powder hopper assembly, mounting bolts, and charge bar from the Upgrade kit available for the Auto disk. (The powder shut off feature is on this larger capacity, round, hopper.) The "square" small capacity reservoir is removed and makes a nice planter.
The "tweak" was to carefully sand down the top "rail" surfaces evenly on the measure's cast housing to fit the hopper assembly better to the charge bar/housing/hopper bottom to prevent leakage and take full benefit of the elastomer wiper ring on the bottom of the upgraded hopper. (As some know the whole assembly is a "sandwich" affair that can leak due to the sloppy fitting, letting fine grained pistol powders leak out on the housing.) This was accomplished by holding the housing in hand upside down while lapping the top "rails" against some 150 grit sandpaper backed up by a hard surface such as a table top. Do not use circular motions but instead a steady to and fro movement.
This prevents rounding off the corners of the rails and results in a FLAT top rail surface. Check the fit often. You usually will not have to take off much. This effectively reduces the height of the rails, allowing a better, snugger fit of the whole assembly when assembled. You just want a semi-snug interference fit so that the charge bar can slide easily while allowing the elastomer wiper on the bottom of the upgrade hopper assembly to glide on the top of the charge bar, preventing leakage at that point.
If you sand off too much, don't worry. Strips of thin metal or aluminum can be made to shim the Hopper assembly back up to proper fit. Check the fit often, though, and you won't have to go this route.
I'm currently using an extremely fine grained ball type pistol powder (WC820) and this slight modification works like a charm. With virtually no leakage at the charge bar/hopper/housing area.
The Micro-just charge bar (comes with the update kit) I have found to be extremely accurate and repeatable, dropping charges of Unique to +- 1/10th grain when set on the higher end and even 4198 stick powder to +- 2/10ths grain. Anything finer is within 1/10th grain or less. Its total Max capacity is about 23 to 24 grs. with a fine powder. The adjustable charge bar is good for down to about 4 grs. Bullseye powder with very good repeatability and operates within a range of charges from there up to about 23 grs. with powders like 296/H110. Utilizing the discs instead of the charge bar, one is able to get down to about 2.5 grs of Bullseye. These discs and charge bar interchange in about 30 seconds. Simply shut off the powder hopper, undue the brass knurled nuts, lift off the hopper assembly and replace the disc or bar.
I just found out that Lee is discontinuing this measure. Par for the course, I guess. However, some may still be available and many still in use hence may be worth modifying for better operation. Wouldn't hurt to e-mail Lee with a desire for them to reintroduce it. This was a best kept secret in my opinion.