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2004, Chevrolet, Silverado, LLY
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Discussion Starter #1
2004.5 Duramax LLY getting a trouble code for open or shorted master cylinder. Spring/Summer blew my brake lines doing 70 on the highway as a driver slammed on their brakes in front of me. Don’t know how I managed it but luckily stopped in the Knick of time. Coasted for 15 miles in traffic “luckily” till I got to my destination to get it towed. Got it fixed with a new set of brake lines and a new master cylinder. Now I’ve been driving around for a while and the light is still on showing the code. Checked the connection and everything looks fine and the truck stops, but every now and then it seams like I lose brake pressure but not enough to leave me with streaks in my pants again. Don’t know if it could be the hydro boost but have changed it myself before around the same time I did the power steering pump with all new steering lines about 7 years or more. The power steering pump is going out the window soon though because I can hear the thing screaming as the truck down shifts after a long highway drive. My fluids are all topped off and no signs of leaks. I usually notice because it’s pretty corrosive and likes to take the very little under coat left off the truck. Any help or similar problem would love an answer!
 

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What is the code? The only electrical component in the master is the fluid level sensor, as far as I know.
 

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LHN...We ARE the Joneses
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Got it fixed with a new set of brake lines and a new master cylinder.
Why would the MC need replacing to solve a blown brake line?

C0201.....Open or shorted master cylinder pressure sensor
That code is ABS related, not MC.
Usually a ground issue with the EBCM.

....The only electrical component in the master is the fluid level sensor,....
True.


Correction

DO NOT REPLACE THE BRAKE MODULE.

Perform the following steps to improve the connection of the EBCM Ground:

1. Remove the EBCM Ground. The EBCM Ground is located on the frame beneath the driver's side door. If multiple grounds are found in this location, the EBCM ground can be identified as the heavy (12-gauge) wire.
2. Clean the area, front and back, using a Whiz Wheel®.
3. Install the ground, then the washer and then the bolt to the frame.

Important: It is important to use the bolts, washers and nuts specified in this bulletin. These parts have been identified due to their conductive finish.

4. Install a washer and nut to the backside of the frame.

Tighten
Tighten the nut to 9 N·m (79 lb in).

5. Cover the front and back side of the repair area using Rubberized Undercoating.
 

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2006 LBZ CCSB Leveled, Hypertech Programmer, 2008 LMM WT, 2008 Gasser WT
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LHN...We ARE the Joneses
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Code has nothing to do with a MC.
 

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2006 LBZ CCSB Leveled, Hypertech Programmer, 2008 LMM WT, 2008 Gasser WT
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Yeah, I just Googled the code and no mention of the MC. Maybe his mechanic is making shit up to sell him more parts?
 

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LHN...We ARE the Joneses
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Mech a nic said:
"We found a nail in one of your rear tires....need to flush the Diff fluid"
:rofl
 

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Discussion Starter #10
They replaced the master cylinder because the brakes were bled dry. Which they were. They told me the master cylinder was damaged and needed to be replaced.
 

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Discussion Starter #11
Thanks for the help will check the grounds. My code reader is saying that it’s and open or shorted MC. I don’t have a mechanic I used one because it was an emergency.
 

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Discussion Starter #12
Update checked the grounds they didn’t look pretty. Clean it up light is still on.
Have been reading more forums and could possibly be abs module or EBCM...
 

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Discussion Starter #14
Different chats on duramax forum. I’m saying that I have checked the voltage on the ABS modulator and getting from the positive on the wire harness to the frame ground negative 12 volts. I know my ground is good and my check engine light is still on. There are different components to the ABS besides the EBCM it looks like. The brain is the EBCM and then there is a pressure modulator.
 
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