I'll keep this as short as possible.

Background
Have a 00 chevy cavalier. Engine runs great at 170,000 miles and have yet to have any major mechanical issues. Have to drive up a really ****ty bumpy road to my home. Lots of snow/ice = slippery and even more bumps. Brakes were "fine" before incident about to be described. However, I noticed the really really slow disappearance of brake fluid. No visible leak, just had to add brake fluid every month. Quite strange I know, but was never able to determine the cause. Always had a feeling the road was damaging the brake lines.

Brand new rotors and pads.

First Instance of No Brakes
Got stuck on ****ty road. Took a long time to get unstuck. During this process, brakes completely gave out (went to the floor offering no brakes whatsoever). Needless to say, I got the vehicle up the road to my home where it now sits. The master cylinder reservoir was completely empty at this point.

Inspection
Pedal goes to floor with no resistance (Obviously, there is no brake fluid in the system). When the car is running, the RPM's jump from 800 (idle) to about 1200 when the pedal is all the way down (possibly overcompensation of the brake booster?).

I placed cardboard under the car. I dumped a large amount of brake fluid into the reservoir. Was unable keep the fluid in reservoir. Examined EVERYWHERE on the car, no leaks. Try pumping the brakes while examining under the cylinder, around the brake lines, and the front calipers. NO LEAKS. So where did this newly added fluid go?

Problem
No Brakes.
Unable to get car to safe location to get wheels off to properly examine the front calipers and rear discs.
No visible leaks.

 

Sounds like a leak in master cylinder and bad booster. I'd say the fluid has been sucked into engine, that's why you do not see leaks.

 

Strange I guess.. what does google say about the condition?

How much cardboard and where did you place it?

Have you even tried to bleed the brakes?

This car has ABS, right?

 

@segfault

Yes! This seems to be the issue. I just spoke with a mechanic friend, he told me the same thing, EXCEPT he said that it's very unlikely that the bad booster has gone bad or is failing. He said, the master cylinder seal has gone and that's causing fluid to improperly move up the intake from the brake booster to the engine.

However, he said the only fix was to replace the master cylinder. Does this seem like a logical conclusion?

@sledge.impy

1. Surrounding the car, I'm 100% positive there are no leaks underneath.

2. Brakes were bled at about 140,000. That was the last time. Doesn't appear to be air in the line.

3. Yes

 

Yes your friend is correct and I was wrong, do not know what I was thinking, too many beers perhaps.
You can get one of those repair kits and restore your master cylinder.