Master Cylinder Bleed

First let me clarify I was having no brake issues. I was simply doing a 200K miles maintenance and was changing brake fluid. So before I started all worked fine.

I pumped out the reservoir leaving about 1/2 inch of fluid in bottom. (Same as on my wife’s car). I refilled with new fluid to top. Replaced cap.

I then bled the lines in order recommended (rear drums first) and they seemed to go well, but when bleeding the front brakes my wife indicated the pedal was very spongy. I finished up as normal but brakes are very soft. After watching videos I decided to bleed the master cylinder. I bought a bleed kit and bled it still installed. Lots a bubbles came out and the pedal hardened up and bubbles stopped. I thought hey it’s fixed. I closed up the MC and cranked the truck and started to drive but problem returned…

I am curious if I can just bleed the lines now? Or should I start over and bleed the MC again before the lines?

I have ABS module in middle of truck under the drivers seat. But remember all was working well before I started so I find it hard to believe that I now have bad parts?


What year is this Chevy?


I am going to guess you own a mid 2000s year truck (you don’t say)… so you should be able to change the brake fluid without exercising the ABS valves. I’ve done my 04 several time that way.

That said, if you introduced a bubble in tbe master, you should crack the master’s lines after bleeding the master. Break the lines loose enough for a small amount of fluid to escape. Use gravity only. If the pedal is still soft, bleed around the vehicle again. Gravity bleeding works as does the pedal or pressure method.

If you have the scan tool to cycle the ABS, use it. If you can’t get the system bled that way, it may take a trip to the mechanic. But I have never needed the scan tool.

Yes it’s a 2007 (New Body) Silverado 1500 2WD.

Ok so is this correct?

  1. Bleed MC lines using gravity. To be specific do I loosen both lines enough to leak and have partner press the brake pedal to force air out? Do they hold the pedal in as I tighten the lines or do they release the pedal before I tight lines? I saw a Scotty Kilmer video where he held petal down entire time as lines were bled and retightened.

  2. After the MC bleed DO NOT crank truck or drive it. Immediately bleed all 4 lines.

  3. I do not have a scan tool .

Here’s the brake bleeding procedure when the master cylinder has been removed, reinstalled, or replaced.

  • Connect a scan tool to the vehicle.
  • Start the engine and allow it to idle.
  • Depress the brake pedal firmly and maintain steady pressure on the pedal.
  • Using the scan tool, begin the automated bleed procedure.
  • Follow the instructions on the scan tool to complete the automated bleed procedure. Release the brake pedal between each test sequence.
  • Turn the ignition OFF and remove the scan tool from the vehicle.
  • Fill the brake master cylinder reservoir to the maximum-fill level with GM approved brake fluid from a clean, sealed brake fluid container.
  • Bleed the hydraulic brake system.
  • With the ignition OFF, apply the brakes 3–5 times, or until the brake pedal becomes firm, in order to deplete the brake booster power reserve.
  • Slowly depress and release the brake pedal. Observe the feel of the brake pedal.
  • If the brake pedal feels spongy, repeat the automated bleeding procedure.
  • If the brake pedal still feels spongy after repeating the automated bleeding procedure inspect the brake system for external leaks.
  • Turn the ignition key ON, with the engine OFF; check to see if the brake system warning lamp remains illuminated.
  • If the brake system warning lamp remains illuminated, Do not allow the vehicle to be driven until it is diagnosed and repaired.
  • Drive the vehicle to exceed 8 mph/ 13 km to allow ABS initialization to occur. Observe brake pedal feel.
  • If the brake pedal feels spongy, repeat the automated bleeding procedure until a firm brake pedal is obtained.


  1. Gravity, no pedal needed. Crack one line at a time and close them as soon as fluid comes out. Removes the bubble from the high spot.
  2. Yes, don’t start it, don’t crank it. Bleed all 4 lines.
  3. Then hope you can get the system bled without it. If not, @testers procedure is the method you’ll need to use, and that will take a trip to the mechanic for you.

Before you do this, look carefully for brake fluid leaks near the ABS modulator. These vehicles are rather notorious for rusting out the metal brake lines down by the ABS modulator.

I’ve personally blown both front and rear circuits at the same time so it completely drained the master cylinder. My after-repair successful fill and bleed was as I described to you without cycling the ABS valves.

Good Luck

Ok guys thanks for help… it’s fixed. No leaks at ABS or anywhere.

Bled the MC with gravity. Pedal tightened up. Then bled each line. I tried using a bleeder pump on first line but air just kept coming in around the valve. I got some help and did the 2 man job where he pressed the pedal, I opened the valve and closed it before he released the pedal. Did that a couple times per tire with no air and just solid stream of fluid. It was more messy but I wiped it up.

Results = my brakes are firmer than ever before, or like new from factory. Very happy!!

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Glad you were successful. Thanks for letting us know it is fixed.

That is how it must be done. You can’t release the brake pedal with a brake bleeder or brake line open without drawing in air.

You can if you use a one man bleeder or just a hose from the bleeder into an elevated jar half full of brake fluid. No second person necessary.

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You would need to seal the bleeder screw threads will silicone paste or air will be drawn in when the pedal is released. Not much air but some customers can notice the difference in quality of the brake bleeding process.

Have not noticed any difference, bleed seemed perfect to me. Would Syl-Glyde work?