Is my master cylinder bad?

i have an 93 intrepid with about 200,000 miles on it. and i don't believe the master cylinder has been replaced. the brake pedal is weak and i tried bleeding it and possibly didn't bleed them enough or right. but when i turn the car on and pump the brakes after pumping them the brakes work better and the car stops better i have excessive pedal travel and when i go on the brakes they get Little firm then sink more. when the car is off and all the vacuum is out of the brake booster it doesn't move much. i notice some corrosion on the brake booster under the master cylinder but there is no leak..... the car was sitting for about 6 years before i got it. my dad bought it new and he doesn't remember changing the master cylinder... hopefully somebody can help , if i had a pressure gauge id check it but i don't