Peel back the boot on the stock master. Inspect for rust and fluid residue. Often the bore in these is compromised by pits in the bottom, or the bore has been honed beyond the specifications.
IMO the modern dual master is the way to go. Also the use of residual pressure checks will retard the introduction of moist air into the brake fluid. The dual master has a diaphragm in the lid which makes it a closed system, the stock mc has an air vent built in where moist air enters.
It's funny, but a new dual master costs less than a new single shot stock master. The dual master bracket and linkage is a one time additional expense. Less than $100. Good luck.
