brakes going to the floor

I have a 74 with 4 drums, no brake booster, and the petal will sometimes lose pressure and go all the way to the floor when I'm stopped at a light. At first I though I lost a wheel cylinder, but the pressure would come back. i checked the fluid, it was pretty low (about half way gone), I filled it up and the car seemed to be ok for a few days than the problem returned, but this time the master cylinder was full. I looked around the back and bottom of the drums (wheels still on) with no signs of leakage. Never had a car with this kind of brake set up, so i'm kind of lost. What do you guys think I should do first?
thanks in advance.

Sounds like the master cylinder has gone bad. The seals in the master cylinder get old and they start letting the fluid leak past letting the pedal drop.

I agree, the M/C has gone away. Sort of scares the crap out of you when the pedal starts dropping as you sit at a light don't it.......LOL. If you mash it harder sometimes it stops going down.

First of all, thanks for the replies. I dont have money right now to do the disk brake conversion, but is it possible to install a master cylinder that has the brake booster? If I do this will i see a difference in my braking performance, and if so are there any other components that need to be changed?
thanks

The booster will help out with braking. You might want to look at all your rubber lines also to make sure they are in good shape. And with drums make sure they are adjusted up.

The fluid is coming out the back of the master past the seal. If you pull the rubber boot off the back by the rod, you'll probably find your missing fluid. Have a rag handy. I know the feeling. Stop at a light, release a little pressure on the pedal and down to the floor it goes. Dave

The booster will help out with braking. You might want to look at all your rubber lines also to make sure they are in good shape. And with drums make sure they are adjusted up.

So can I just buy a master cylinder for a 74 with power brakes, swap and hook up vacuum or do I need to change/modify anything else?

So can I just buy a master cylinder for a 74 with power brakes, swap and hook up vacuum or do I need to change/modify anything else?

You'll need a adjustable porportioning valve too. search the site for this, I think there's probably a thread on that. Four wheel drum brakes with power brakes and the stock valve are a little too touchy.

Only buy the master for power brakes if you're putting the booster on at the same time. And you will need the correct prop valve when you do. If you're not putting the booster on at that time, just get a regular master for the car and go drive it. I don't know if the prop valve is different for drum/drum and disc/drum. Anybody? Dave

The proportioning valves are different between disc/disc, & disc/drum. There isn't a difference between manual or power. The power booster does nothing for the proportioning, it only adds pedal assist.

So then there is a difference between the power M/C and the manual M/C?

Yes, power M/C is operated by the booster. Buy the M/C and booster as one unit and you will be set to go.

Yes, power M/C is operated by the booster. Buy the M/C and booster as one unit and you will be set to go.
Thats what I wanted to do originally, that is until I saw how much a new Booster costs, so I'm thinking new M/C with a Junk yard booster, can someone give me some year/models of some cars that can be used, or do I specifically need one out of a 74 nova custom?