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Old 12-09-2017, 11:00 PM   #1
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Default Master Cylinder / ABS Module

I have a 2008 Astra XE 5-speed. 89k miles. Love the car. A week ago while changing the drivers side half shaft (torn boot) I used a C-clamp to press in the caliper piston to remove the caliper to do the shaft replacement. Then I decided to do the front brakes while I was in there, so pressed in the passenger side as well. I read later in the Haynes manual that I should have opened the bleed screw before pressing in the piston to avoid forcing the brake fluid back up into the ABS module & MC. News to me after doing brakes for 30 years. My bad.
After job was done I had very weak, spongy front brakes. Pumping the pedal a couple times would build up pressure, but holding that pressure on the pedal would result in a slowly sinking pedal. Per other posts I read, this was classic behavior of a messed up master cylinder. Today I replaced the master cylinder and bled everything. It still has the same behavior. So apparently the issue wasn't the MC. I guess I must have blown seals or something in the ABC control unit.
Before I spend $400 on a new ABS control unit, can anyone confirm this is the likely issue? Any other thoughts?
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Old 12-10-2017, 12:14 AM   #2
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2003 L-Series 3.0L Sedan
Default Re: Master Cylinder / ABS Module

If hydraulic brakes haven't changed and abs added to enhance braking in extreme situations, you haven't missed anything other than staying informed as braking systems evolved from basic hydraulics - adding abs then traction control and last, stabilitrak. All enhancements with electronics in the abs module with a high pressure pump. The basics you already know, no engine running while flushing or bleeding procedures are performed. This accomplishes two things; (a)abs is never powered up and activated if any wheel is off the ground and wheel speed is sensed to allow abs to react at the wrong time during flushing/bleeding procedures, (b)all abs valves are closed, making abs invisible to normal everyday braking. Correct me if I'm wrong but abs/tc/stabilitrak is always invisible to the hydraulic system and will not affect basic braking in the event abs power is lost, continuous abs self monitoring detects a fault and disables itself and if someone somehow manages to do all the wrong things during brake maintenance and manages to have air enter the abs unit. Even is air was introduced inadvertently to the abs unit, the first application of abs, tc or stabilitrak with the electronics detecting incorrect wheel speeds will automatically disable itself from interfering with braking, returning basic braking to the driver. In effect, failure of abs/tc/stabilitrak defaults back to basic brakes.

While many presume their abs module failed, more misinformation is given along with presumptions of abs failure, compounding repairs. The key to understanding abs is to never idle the engine during brake fluid replacement. All brake fluid flushing and bleeding procedures are the same whether abs is installed or not. Since abs valves are closed with engine off, abs not powered, normal brake fluid replacement is the same as if abs were never there. Even when replacing the brake master cylinder.

It's presumed you're familiar with master cylinder bleeding before bleeding brake lines. Review all you did carefully. Somewhere during replacement of the master cylinder, bleeding it and flushing lines for new fluid, air may have been introduced. In case you forgot, left side drive with brake master cylinder on the left side of the car uses brake flushing/bleeding in this order - right rear then left front, left rear/right front to allow fresh fluid into the farthest brake line to the nearest in a dual diagonal brake system. Pressure, vacuum, two person or single person, brake fluid replacement remains the same procedure.
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abs, brakes, master cylinder

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