Weird brakes

I drove the car to work today like usual with no problems and during lunch while putting new bulbs in the cluster I noticed the brake light to be on. Sure enough, it had a very soft pedal and it had to be pumped up. I drove it around and sometimes it has a firm pedal and it stops fine, other times it's very soft and the light comes on, but it still stops halfway decently. Also, when the light comes on, it will stay on until I tap the pedal. My thoughts are on the master cylinder, but I'm not too sure. I know I need a new gasket to go on top of it because that leaks, but I wouldn't think that would cause the braking pressure to be low and then its random at that. This car is a slant six and it's power brakes. From how things look, it's the original master cylinder, booster, and front calipers from '76, and I put new wheel cylinders in the back about a month ago because I noticed the drum side of the master cylinder to be low. As far as the rubber hoses, they look newer than the car, but I don't know how old they are. Is there anything I should look at beyond the master cylinder and hoses? Or should I just get them and hope that fixes it? I'm great with engines and a little bit of electronics, but hydraulic systems, not so much, but one of those two is bound to be the culprit by my thinking. Thanks for any help.