Brake Issues

[abeemanator]

I have a 1984 Toyota 4x4 pickup, solid front axle, disk on the front, drum on the rear, and I was out driving today, and my brake light came on, so i checked the fluid and I was below min, so I filled it back up and off I went, but I noticed the pedal would be soft, to a point, then all of a sudden I had alot more braking power, but when i let off and pushed again, the brake would work fine.(It did this to me a few weeks ago, and I had enough fluid, then the brakes seemed to work fine after a while) I am just wondering what could cause this. I had an issue on my mitsu after the master cylinder died on me, and it would not push any pressure though. I am a little afraid to drive it much more because I don't want the brakes going out in traffic (Emergency brake cable is broke, so no E-brake). My friends dad said I could have a leaking wheel cylinder, and when I press and hold the brake, it will very slowly begin to fall to the floor. Any info would be appreciated