using drum MC on front disc setup

The reservoir is to small to supply the calipers with fluid. You will run out very quick. And if you add fluid the next time you put pads on you will push it out the top.

Rule of thumb is when the master is low on the big/rear side you need pads. A drum brake master will run out before the pads are worn.

Newer style masters share the reservoirs and is what I used to replace my original disc Master in pic below