Brake booster causing low brake pedal

Tiny
TOOKIEN
  • MEMBER
  • 2007 CHEVROLET TRAILBLAZER
  • 4.2L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 150,000 MILES
Hello,

I have issue where:

Brake pedal sinks to the floor when pressed.
When car is turned on, when brake pedal depressed a hissing noise comes from the brake booster, as well as rough idle.

(I have all the symptoms of failed brake booster except for the fact that the brake pedal is soft and not hard).

Bleeding brakes (on each wheel) does not change anything.

I tried two master cylinders. One original and one used (got from junkyard). Same problem with both.

Both master cylinders were successfully bench bleed before installed.


For more testing I also capped the master cylinder outlet brake line ports to see if the firmness of the brake pedal changed.


Still the same problem.

Brake pedal just sinks to the floor.

Which leads me to believe it is in fact the brake booster.


Or, it could be that both master cylinders that I tested with were bad.

Thanks very much for any help!
Do you
have the same problem?
Yes
No
Thursday, June 21st, 2018 AT 6:13 PM

5 Replies

Tiny
ASEMASTER6371
  • EXPERT
Good morning.

Yes, the vacuum leak from the booster indicates it has failed and is causing the engine to run rough and will set a lean code shortly.

As far as the brake pedal, you have air in the system. It needs to be bled but you need to bleed the ABS module. That has to be done with a scan tool as it needs the valves commanded open to get the air out of the unit. If you do not have a scan tool then any shop can do this for you.

Roy

Antilock Brake System Automated Bleed Procedure

Two - Person Procedure

Important:
Use the two-person bleed procedure under the following conditions:

- Installing a new Electro-Hydraulic Control Unit (EHCU) or new Brake Pressure Modulator Valve (BPMV).

- Air is trapped in the valve body.

Do not drive the vehicle until the brake pedal feels firm.

Do not reuse brake fluid that is used during bleeding.

Use the vacuum, the pressure and the gravity bleeding procedures only for base brake bleeding.

1. Raise the vehicle in order to access the system bleed screws.
2. Bleed the system at the right rear wheel first.
3. Install a clear hose on the bleed screw.
4. Immerse the opposite end of the hose into a container partially filled with clean DOT 3 brake fluid.
5. Open the bleed screw 1/2 to 1 full turn.
6. Slowly depress the brake pedal. While the pedal is depressed to its full extent, tighten the bleed screw.
7. Release the brake pedal and wait 10-15 seconds for the master cylinder pistons to return to the home position.
8. Repeat the previous steps for the remaining wheels. The brake fluid which is present at each bleed screw should be clean and free of air.
9. This procedure may use more than a pint of fluid per wheel. Check the master cylinder fluid level every four to six strokes of the brake pedal in order to avoid running the system dry.
10. Press the brake pedal firmly and run the Scan Tool Automated Bleed Procedure. Release the brake pedal between each test.
11. Bleed all four wheels again using Steps 3-9. This will remove the remaining air from the brake system.
12. Evaluate the feel of the brake pedal before attempting to drive the vehicle.
13. Bleed the system as many times as necessary in order to obtain the appropriate feel of the pedal.
Was this
answer
helpful?
Yes
No
Friday, June 22nd, 2018 AT 4:32 AM
Tiny
TOOKIEN
  • MEMBER
Thank you so much for the advice.

A question though:

The ABS module is causing the low pedal even though I disconnected the brake lines from the master cylinder (capped the brake line ports) and have the exact same symptom (ie: low pedal)?
Was this
answer
helpful?
Yes
No
Friday, June 22nd, 2018 AT 5:06 AM
Tiny
ASEMASTER6371
  • EXPERT
You need to replace the booster as a start. Then you need to bleed the system.

The booster is a confirmed failure and is the first thing that needs to be replaced. Then put it together and bleed it out.

Roy
Was this
answer
helpful?
Yes
No
+1
Friday, June 22nd, 2018 AT 5:13 AM
Tiny
TOOKIEN
  • MEMBER
I forget to reply on the conclusion (which was a while ago).

The issue was an adapter seal in the brake booster fell out when I took off the master cylinder.
I had no idea anything fell out until I bought the new brake booster and saw the difference.

This adapter attaches to the end of the brake booster rod which pushes against the master cylinder piston.

This adapter (rubber/plastic) seal was responsible for the hissing noise/vacuum leak coming from the brake booster.

I couldn't find the adapter and simply replaced the brake booster along with a new master cylinder.

Everything was fine after.

The old master cylinder was definitely shot as it didn't allow me to bleed out the brakes, the internal seals must have gone bad.
Was this
answer
helpful?
Yes
No
Saturday, December 22nd, 2018 AT 2:05 PM
Tiny
ASEMASTER6371
  • EXPERT
Good job.

Roy
Was this
answer
helpful?
Yes
No
+1
Saturday, December 22nd, 2018 AT 3:19 PM

Please login or register to post a reply.

Recommended Guides