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Brake Master cylinder possible leak

Tiny
HD2424
  • MEMBER
  • 2003 CHEVROLET SILVERADO
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 246,000 MILES
I installed a new computer and engine harness; the truck was out of commission for a couple of weeks while doing all that installation of computer and engine harness. Truck is running now. As I was finishing up the computer, I noticed the Brake Master Cylinder was very low. I added brake fluid, but the brakes are spongy. I checked all brake calipers, but none seemed to be leaking. Do I need to change my master cylinder? Or where could all that fluid have gone?
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Thursday, March 31st, 2022 AT 7:03 PM

13 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
I need you to try something. Start the engine and press the brake pedal and hold pressure on it. Does it slowly go to the floor?

Also, was the brake system opened for any reason?

Let me know.

Joe
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Thursday, March 31st, 2022 AT 8:43 PM
Tiny
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Okay, will do. I just wanted to add something. The brakes were fine before I started doing engine harness.
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Friday, April 1st, 2022 AT 3:56 AM
Tiny
HD2424
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  • 191 POSTS
Okay, I started truck pressed brake pedal slowly and it does go all the way to floor.

And have not done any work opening up brake system.
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Friday, April 1st, 2022 AT 7:24 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
If there are no leaks, the master cylinder is likely the problem. Take a look at this link:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

If you feel it mirrors what you experience, here is a link that explains in general how it's done:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Because you are replacing the master cylinder, chances are you will need to perform an automated bleed on the ABS module. That will require a scan tool that can communicate with the ABS module.

The attachments below are the directions specific to your vehicle. You will need to manually bleed the system. I attached the directions for that as well.

One other thing, make sure to bench bleed the new master cylinder as explained in the link I attached.

Let me know how things turn out for you or if you have other questions.

Take care,

Joe

See pics below.
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Friday, April 1st, 2022 AT 1:51 PM
Tiny
HD2424
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  • 191 POSTS
Okay, I understand all of that, but I don’t understand where the brake fluid went. The only possible place is in brake booster. Do you think I ought to change booster too if it leaked in there?

Thanks for all your help
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Friday, April 1st, 2022 AT 2:24 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

If the fluid leaked into the booster, just try to clean it out. It should be fine. The seal at the rear of the master cylinder may be leaking and as you suspect, that is where it would go. However, that shouldn't cause a problem.

Just for curiosity, if you refilled it, does the level drop if you keep pumping the brakes?

Let me know.

Joe
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Friday, April 1st, 2022 AT 7:19 PM
Tiny
HD2424
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Yes, I believe so. Today I sprayed brake cleaner all around back of master cylinder and I filled master cylinder going to see if it brake fluid leaks out without pumping brakes, I’ll keep you posted.
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Friday, April 1st, 2022 AT 7:49 PM
Tiny
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Okay, update like I said earlier I filled master cylinder up and saw no noticeable leak on outside of master cylinder. I pumped the brakes and fluid in master cylinder goes down.
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Saturday, April 2nd, 2022 AT 11:11 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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If nothing is leaking under the vehicle, it sounds like it is the master cylinder doing what you expected.

That is where I would start. Before disconnecting any of the brake lines, loosen the master cylinder and gently pull it away from the booster. Do this as far as you can without bending or damaging the brake lines. You should be able to pull it away far enough to see if that is where it's leaking.

Let me know what you find.

Take care,

Joe
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Saturday, April 2nd, 2022 AT 8:57 PM
Tiny
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Will do.

Thanks
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Saturday, April 2nd, 2022 AT 9:04 PM
Tiny
HD2424
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Hey Joe, got it figured out, took off master cylinder and it was bone dry. So, I did some more looking; I pumped the brakes several times and found a puddle, it was brake fluid that ran along the rear end. It had snapped off. I fixed it for now with compression fittings for now until I can get to junkyard to get one.

Thanks for all your help.
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Sunday, April 3rd, 2022 AT 3:38 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

I'm glad to know you found the problem, and thanks for the update.

Remember, if the steel line rusted there, it is likely bad in other locations. So, it is a good idea to repair it.

Regardless, you are very welcome. Please feel free to come back anytime in the future. You are always welcome here.

Take care,

Joe
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Sunday, April 3rd, 2022 AT 7:36 PM
Tiny
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Thanks Joe.
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Sunday, April 3rd, 2022 AT 9:06 PM

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