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2004 GMC YUKON DENALI: Approximately 120,000 miles The

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2004 GMC YUKON DENALI: Approximately...
2004 GMC YUKON DENALI: Approximately 120,000 miles
The battery died on the vehicle and had been sitting for approximately 1 year. Prior 2 the dead battery, the vehicle ran very well. I had to take it off the road for financial reasons. The only issue was a Oxygen Sensor. the battery and it started right up.
The problem, which was not a problem prior, is that the brake pedal and the emergency brake both go right to the floor and are inoperable. Does this have anything to do with with the dead battery and/or the time it was sitting? A electrical or computer reset issue? I replaced all fuses under the hood and at the inside panel that are related to the brakes. I am sure this has nothing to do with it but I want to give you as much info as possible, the passenger rear tire has gone flat and began to dry rot.
Any thoughts?Also, re:oxygen sensor. I replaced the sensor and had not been able to get it to reset, drove several hundred miles. I wanted to replace the sensor and the entire wire. Where can a get a diagram that shows where the wires run to?
Thank you and I appreciate any info you can share.
Submitted: 4 months ago.Category: GMC
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Answered in 4 minutes by:
3/9/2018
GMC Mechanic: Jared D, ASE Certified Technician replied 4 months ago
Jared D
Jared D, ASE Certified Technician
Category: GMC
Satisfied Customers: 2,277
Experience: Shop Supervisor at County Fleet
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Good morning,

To clarify, the brake pedal falls all the way to the floor and the brakes don't seem to be working at all right?

Do you live in an area where rust is common?

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Customer reply replied 4 months ago
when I touch the brake it goes to the floor and does not work at all. Including the emergency brake.
I don’t live in a area where rust is common. There is rust under the vehicle but I wouldn’t say excessive.
GMC Mechanic: Jared D, ASE Certified Technician replied 4 months ago

I would probably inspect the brake system for signs of leaking fluid. Typically when the pedal falls like that it's due to a fluid leak somewhere.

Also, how does the fluid level look?

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Customer reply replied 4 months ago
The fluid level is fine. And no leaks under the truck.
GMC Mechanic: Jared D, ASE Certified Technician replied 4 months ago

In that case the master cylinder has likely failed. This is not common, but it's possible. If you have an assistant press the brake wile you watch the reservoir can you see the fluid move at all or does it pretty much stay still?

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Customer reply replied 4 months ago
I will try that today. Sorry for the delay.
Customer reply replied 4 months ago
I checked the reservoir and it was empty. When the brake was pressed, I could see the fluid at the bottom of the reservoir moving.
When I looked at the fluid before, I assume I thought it was full by looking at the outside, which does appear to be at the correct level due to previous fluid residue.
I checked underneath and don’t see any oil. I assume it had to go somewhere. It is parked on a grass surface and zip assume overtime I wouldn’t be able to see and fluid on the ground.
Should I refill the fluid?
GMC Mechanic: Jared D, ASE Certified Technician replied 4 months ago

I would fill it back up with fluid and then see if you can see it come back out somewhere.

Also, look under the dash where the brake pushrod goes through the firewall. Sometimes it will leak fluid there too.

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