Always make sure the fluid is not too low but do not add too much unless all the brakes are new.
The pedal should not have to be pumped to be firm, if so there is air in the system.
The best garrantee that the master cylinder is responsible is if the pedal sinks while applied with the engine running, or if the pedal always goes lower when applied slowly.
I would check that the drums are adjusted properly first, they can take up a lot of pedal travel.
If the brake fluid is original you are well overdue for replacement. Since the clutch and brakes are connected this must be done carefully.
