If you use the type reservoirs and especially caps that Girling supplied for decades do yourself multiple favors and buy some aftermarket silicon rubber dirt/air/water seals.
They are especially useful on original style clutch master cylinders that gets road splash. Those little breather holes in the Girling caps let the world into the reservoirs pretty quickly. For me they ended the twice a year flushing of brakes and clutch and mushy clutch in between flushes. Exhaust heat also doesn't seem to affect the brake master cylinders like it use to when atmospheric water was being pulled into the reservoirs every time the engine bay cooled off.
The Girling rubber cap seals come out; as do the plastic strainers in the steel reservoirs.