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Unread post by maddjack » Fri Jun 29, 2018 7:26 pm

2005 Trailblazer, has been sitting for a very long time, both front calipers seized, replaced front calipers, brakes all the way around, and a new master cylinder. I am old school, I haven't been brake certified since 1981, which means I know nothing of ABS.

I have no air in the lines, awesome fluid flow, Rock hard pedal with engine off, start engine, pedal will go almost to floor, with a sound of lightly rushing air under dash. All test for vacuum booster indicate that it is fine, but I'm beginning to have my doubts. At the same time the symptom isn't what my memory of a vacuum booster acts like when it's bad. As I recall with a bad vacuum booster you'll get a solid 2 foot brake pedal to stop the car. I'm 61 years old on disability, I've done this sort of work all my life, but the sun and the heat are killing me. Please help.

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Unread post by carriedi » Fri Jun 29, 2018 9:11 pm

take the vacuum line off the booster and plug it off. this way there is no vacuum just like when the engine doesn't run. Pump the brake pedal so you know all the vacuum is gone. Then start the engine up. Press on the brake pedal now. Does it still feel up on the top and hard to push? If the abs is the problem it would probably not feel the same as when the engine wasn't running. If the brake pedal feels just like it did before when the booster was hooked up but the vacuum pumped out of it then it is probably a bad booster. If the pedal act differently after you start the engine and have it running then you may have a problem with the abs. Scan for abs codes and look at your data stream to see what's happening.

ABS, normally, doesn't affect your regular braking. It just sits on the side until your wheel speed looks like a lock up is going to happen and it releases and reapplies the pressure to the wheels that are locking up.
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