Actually they are tiny Philips head bolts, but everyone calls them screws, so that's what we will call them here. Those little screws in the top of the reservoir of your brake or clutch master cylinder are subject to corrosion in their location, and you will often find them seized when you try to remove them to check the brake fluid.
To avoid stripping the slots in the head, make sure you are using a proper sized, good quality screwdriver that fits the head exactly. Put a drop of penetrating oil on each screw and with the right pressure and torque you might get lucky and get them to turn. If they won't budge, an impact driver might be your next step, but there is a chance that the impact could break the cast aluminum housing of the master cylinder.
The best method, short of drilling the screws out, is a simple trick we discovered using an automatic center punch. This is a little spring-loaded punch that snaps an impact to the punch head when pushed. It is available from most tool suppliers for a few dollars. This trick will work even if the screw heads are stripped.
Place the punch head at the outer edge of the screw and make a small dot. Then add a drop of penetrating oil, and angle the punch in the direction you want the screw to turn. Try not to chew up the head too badly, and continue snapping the punch in the dot, angled a bit in the right direction, until you see the screw turning. The impact helps the oil penetrate into the corroded threads, and provides a positive turning force on the screw when done correctly.
If that method fails, your next step will be to drill dead center into the screw head until the head falls off. Don't use too big a drill bit though, you don't want to damage the cover. Once the head comes off, you can remove the master cylinder cover and usually the screw shaft can be removed with small vice-grips.
All these tips, and many more, can be found in The Motorcycle Handbook along with more complete explanations on many aspects of motorcycles and motorcycling. 63 illustrated chapters of practical, down-to-earth information you won’t find anywhere else! Just click on The Motorcycle Handbook link for more info and some excellent reviews.