1991 Isuzu Amigo Brakes driving me nutts

Brakes problem
1991 Isuzu Amigo 4 cyl Four Wheel Drive Manual? Miles

First let me say Merry Christmas to everyone out their and thanks for allowing me to join the group! My dad bought me my first vehicle and you guessed it it's an Amigo! My dad has always had jeeps so I'm kind of a 4x4 kinda girl. It was running like a top but recently the brakes have been acting funny. My dad replaced the master cylinder but it keeps doing the same thing.I can pump the brakes and get a good pedal and it will hold up with no fading but it goes close to the floor between traffic lights. My dad thinks vacuum from the motor is the cause and is leaning towards the booster being bad but is not sure. The one way valve inside the vacuum hose going to the booster seems to be functioning right and you can head the suction release when you remove the hose from the booster. Any Amigo people out there have any suggestions that might help him out? I wanna get back on the road but not till the problem is resolved. Thanks in advance. Brittany Rosenzweig!