I don't think you have a master cylinder problem. . Check the pedal return spring and linkage for rust. Spray all linkage points liberally with WD40.
1998 Pace Arrow 35 ft. F53 Ford V10 2014 Honda CRV toad
32 years mechanic at Delta Air Lines 15 year motorhome service manager. 3 popups....2 travel trailers....5 motorhomes....loved them all.