help with brakes ls1 fc

[SpunRX5.7]

Everything brake related is factory. 86 gxl 4pot. I noticed yesterday it seemed I had to push a lot further on the pedal to get brake response. It was late and dark so I waited until this morning to check it out. The mc was close to empty. I topped it off a month or two ago just for the hell of it while doing an oil change, so I know it WAS full. I took all the wheels off and inspected the lines/bleeders. Everything is dry. I searched the "club" and here, read something about the fluid can leak out the back of the mc into the booster? The strange thing is the brakes seemed fine when I parked it, then bad after sitting.  Brakes still work but they slowly fade and I have to keep applying pressure to stop at the same rate. How can I tell if the mc has a leak?

Thought I ask if anyone has been through this or if this is a common fc poblem and I overlooked something.

Anyways, thanks for the help 

[dream36realms]

actually your symptoms indicate a leak. the fact that it fades means it could have a leak. there should be a point where the pedal sits even if it does take more pressure to squeeze out a lil more.

[clocker]

It is possible for the MC to "leak into the booster"- you can normally tell by drips below the MC on the face of the booster.
If you swipe your finger under the MC you may be able to feel it.

If not, the falling fluid level certainly means there is a leak somewhere and you'll just have to track it down.

[DDv8]

There may not be drips on the booster but the paint will bubble and the frame below may have blistered paint. Mine just went low for second time in six months, blistered paint and damp/oil on the booster.

[clocker]

If you can't see anything on the booster face but still suspect that's where the problem lies, take the two MC mounting stud nuts off and pull the MC out a bit...that'll show you right away if the fluid is leaking there or not.

Have a rag handy, just in case.