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Brake system help on 2009 Focus SEL

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I need some help or advice. Driving home early in the week when my brake pedal went almost to the floor before the brakes began to take hold. Car also pulled to the one side and took a long distance to stop. My first thought was that there must be a leak in the brake system and the dual braking system had locked out the side with the leak, leaving the other side functional. That's what it is designed to do. Got home and inspected everything. No leaks anywhere.  Brake fluid reservoir is full and no leaks. No code for ABS or traction control system, and no warning lights on the dash. I don't hear or feel the ABS motor kicking on, but all fuses listed for the system check as good. My only other thought would be that an inner seal in the master cylinder has gone bad, allowing the fluid to return the the reservoir instead of pushing it into the caliper or wheel cylinder. I've read that sometimes a valve will stick in the ABS causing this same symptom, but I would assume that should trigger an ABS fault code; or am I wrong about it setting a code? Strangest brake system I've worked on in a long time; disc on front and drums on rear, but has ABS and traction control. The way this pulled when trying to stop felt like the early "dual diagonal" brake systems rather than the current systems of the dual front / rear split. Any ideas or thoughts as to what else the problem could be would be greatly appreciated.

I should also let you know that I've spent almost 50 years in the car business and was factory certified in multiple import lines. However, that was in the "70's and early 80's, before ASE became well established; so I'll readily admit that I'm not completely familiar with these newer ABS systems. I'm sure I have the tools and skill necessary to repair it, but I could use a little more knowledge of these systems, especially what to look for and where to look. I'm hoping some of you have had more experience with these and are willing to share your knowledge.  

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Sticking isolation valve in the hydraulic control unit for the ABS would not set a code but could cause a low pedal. Fusions had that problem 06-09 and the HCU needed replacement. 
my first suspect would be the master cylinder. If you have some plugs to screw into the ports of master cylinder where the lines are and the pedal still sinks then it’s bypassing internally. Brake booster won’t cause a low pedal, rather a hard pedal or hissing noise. 
isolating the master cylinder first is the first step I would take. It’s basically a standard brake system with just a hcu to release brake pressure on a wheel or wheels during an ABS event. 

Edited by fordtech1

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