Hi, I have a 2007 Ford Focus and 'bout a month ago, the brakes lost compression when applied. The mechanic replaced the master cylinder and we thought everything was ok. After driving it around we noticed the brakes are less sensitive then before. Prior to this, lightly taping/pressing the brakes would slow the car. Now you must press more firmly and with a bigger stopping distance. Additionally, if you press very hard or very suddenly, it feels like the roughness of the road is transmitted to the foot. It’s a bit choppy too and the car won’t stop for several seconds. After 2-3 seconds it does stop. This happened three times in two weeks. It’s never occurred before. When this happens to my husband, he pumps the brakes to correct it, and when this happens to me, I keep pressing down 'till it works. Could the anti-lock brakes be acting up? We brought it to the mechanic and so far he hasn’t found a problem.
He should remove the master cylinder again and look behind it inside the brake booster and see if it’s full of brake fluid. Might need a dip stick of sorts to check this. Brake fluid will rupture the diaphram (always a problem) and cause a loss of braking “power” assist.
I suspect that maybe the master cylinder wasn’t bled properly. They need to be throughly bench bled before installing to get all the air out.
Another thought is if it is a remanufactured master cylinder, not new, it could still have a defect. I went through a rash of these until we switched suppliers.