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Old 09-18-2012, 08:18 PM   #1
geartooth94
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Default Mushy brake pedal: master cyl. or power booster?

Hi guys.

I recently did a search on this topic, but couldn't find any brake booster tests beyond pumping the pedal when the car is off to deplete vacuum. I should then have a hard pedal that doesn't move more than 1/2 - 1 inch.

When I do this test, the vacuum does deplete, but I am left with a stiffer pedal that goes down effortlessly half of its travel, and then gets harder to push. I can still bottom out the pedal though with a bit more leg force.


Back story:

Replaced RR wheel cylinder, and bled that line at the cylinder with the engine running, (I know, stupid mistake.), and with the entire butt end of the car lifted at the rear in the middle of the rear crossmember.

Then I took the car to a local chain which I trust and had them bleed all 4 lines at each wheel. Pedal got a little better, but still has about 1/2 its travel with no stopping response. The brakes do bite, but stopping takes longer.

What I want to know is: are there any other tests I can do with the booster before confirming it good / dead?
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1996 Dodge Avenger ES: 2.5L V6
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Old 09-18-2012, 09:13 PM   #2
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2003 L-Series 3.0L Sedan
Default Re: Mushy brake pedal: master cyl. or power booster?

Quote:
Originally Posted by geartooth94 View Post
......When I do this test, the vacuum does deplete, but I am left with a stiffer pedal that goes down effortlessly half of its travel, and then gets harder to push. I can still bottom out the pedal though with a bit more leg force......
1-Once the vacuum is bled with several brake pedal applications (engine always [U]off[/U)), a good brake system won't allow the pedal to sink, even when standing on the pedal.

You're master cylinder is done. Replace it or learn to rebuild it but expect a big learning curve if you don't know hydraulics. Your life is in your hands if you make any mistake and suddenly find yourself with no brakes when its needed. You'll also have to consider worn out brake hoses that bulge from pressure you can't see.

2-If there are absolutely no fluid leaks anywhere in the brake system and the pedal goes to the floor, the master cylinder isn't working anymore. The piston and seals aren't preventing fluid from leaking past instead of generating several hundred pounds of hydraulic pressure - the pressure that's used to force brake shoes into drums and/or squeeze brake pads against rotors.

The brake vacuum boost unit uses engine vacuum to multiply your leg power to generate more hydraulic pressure than without power assist.
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Old 09-19-2012, 11:25 PM   #3
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Default Re: Mushy brake pedal: master cyl. or power booster?

no brake fluid leaks and no brake hose bulges that I can see.

Will get to master cylinder replacement and report back.
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