Technical Where does this stuff go??!

My hot rod has Master cylinder issues for sure. Soft pedal/low pedal...I wonder every time I hit the brakes is it gonna stop! Front discs/rear drums. Maverick dual bowl MC.
It has been under the car cover for about a month. Today...the pedal went to the floor. Checked the MC and the rear bowl(front discs) was almost empty. The strange thing is....where did the fluid go?
NO puddle on the floor.....outside of MC was dry....no leak from the calipers!!
So...where did the fluid go? It doesn't 'evaporate' does it?
Filled the bowl and topped off the front bowl and soft/low pedal cam back enough to stop the car. Since I have Chevy calipers I am going to try an S-10 MC....but, where did the fluid go?!!
6sally6

 

No it doesn't evaporate. If it's actually losing fluid, it's leaking somewhere. Power brake booster?

 

I have an off topic mid 90s chevy truck that the hard brake line that lays in the frame rail was getting pin holes, and the frame rail held quite a bit of brake fluid before it actually started dripping. Wheel cylinders were dry, it took a hard look in other places to find brake fluid.

 

It's doubtful that your front pads wore that much in that short of time so we can pretty well eliminate that.
One thing to look for is a nearly undetectable leak in the front brake lines. Go all though the brake lines inch by inch and pay serious attention to every connection. I had a pretty well hidden fitting leaking on one of my wife's ot rigs a few years ago that drove me nuts and was doing the same thing. There was a bad flair on one of the tubes that was letting a very tiny amount of fluid leak out and letting it suck air back in.
You might want to start by unbolting the master cylinder and slipping it far enough forward to see the back of it and see if it is leaking though. That would be the first step in checking everything. If it is on a booster it probably is a 5 minute process from start to finish to check it if you are a bit slow or it is hard to reach and you shouldn't have to take the lines loose just to check it.

 

I've found several times in maintaining a fleet of trucks the fluid sometimes leaks from a very small leak in one of the drum brake wheel cylinders. So small the heat from the brakes evaporates the teeny amount of fluid and nothing gets wet. After a while the shoes will get glazed and soon after the leak will be too much for the heat to evaporate.

 

if its leaking into the booster, vacuum will suck it into the motor, or fill the booster, no?

 

Yes that can happen...
If the rubber diaphragm is bad, it gets sucked into intake and burned.