So when I idle after turning my car on it idles at around 1000 RPM UNTIL I brake, then everytime I brake the rpms drop more and more until the car literally feels like it's stalling every time I brake. I recently had all brake rotors, calipers, pads changed and the mechanic said no issues with master cylinder or leaking. At the same time the cars breaks literally go in all the way and the car won't stop, just randomly the brakes go out. What the hell is going on? Is it the booster? Alternator? Battery? I'm so lost.

 

If the brake pedal is dropping after you've come to a stop then either the master cylinder is leaking internally or you have an external leak in the system. You must immediately stop driving and fix this problem or you run the risk of completely losing the brakes while driving and possibly dying as a result. Stop going to that mechanic. He clearly isn't taking this serious enough. Find another mechanic with good reviews on Yelp.

 

It's not like it becomes completely loose and drops, it just hits the floor, I'm honestly guessing it's the booster. I ordered a replacement, I'll see if that fixes it.