Soft brakes when engine running

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  #1  
Old 05-12-20, 12:58 PM
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Soft brakes when engine running

I have just replaced a master cylinder in a 2002 Chevy Trailblazer LTZ. The new cylinder made some difference but I am still unsure what may still be an issue. My 2 mechanic friends are telling me I likely got a bad cylinder but here is the issue.

Before replacing the master the pedal would go to the floor even when engine not running. Now with the new cylinder the pedal is very firm when engine is not on but when it is started the brakes are still extremely spongy and with effort the pedal goes all the way to the floor.

All lines have been vacuum bled
Actually 2 rounds of bleeding have been done
all calipers checked and working properly
Brake pads are not new but have 50% left
All wheel bearings are tight
No warning lights or codes show up

I have not gotten into the ABS yet but the brakes just went out completely before while driving, fine one minute and zero brakes the next. Serious **** your pants moment. I do not see anywhere that the ABS would do this that quickly.

I am mainly curious how I can have such a strong peddle with the engine off but super spongy when at idle.

Thanks in advance for any advice.
 

Last edited by the_tow_guy; 05-13-20 at 05:01 AM.
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  #2  
Old 05-12-20, 03:32 PM
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Bad/leaking brake booster or vacuum line!
 
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Old 05-12-20, 03:47 PM
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Did you bench bleed new M/C? Was a common problem on M/C's around that time. We always bench bleed them before we installed. Used to send some tubes and fittings to do this.
 
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Old 05-12-20, 04:12 PM
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You still have air in the system.
 
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Old 05-12-20, 10:14 PM
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the vacuum booster is just assisting you to push the pedal down much further, normal for vehicles to get a lower pedal with the engine running but if its still going to the floor would also suspect you still have some air in the system would try bleeding again and maybe get a helper to pump the brakes up and hold them, instead of vacuum bleeding.
 
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Old 05-13-20, 01:50 AM
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Now with the new cylinder the pedal is very firm when engine is not on
So this indicates that there is no air in the system!

with effort the pedal goes all the way to the floor.
OP states "WITH EFFORT" the pedal will go to floor indicating the pedal is hard and not functioning. Sounds like a booster/vacuum issue!
 
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Old 05-13-20, 10:04 AM
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I am with leaking booster bunch. Diaphragms on them do go bad.
Also, when reinstalled, push rod on MC has to be set per spec. If not, all kinds of things happen. How do I know....
Not uncommon to have bad aftermarket part either.
Otherwise, oddly sounding, poor brakes relate to poor compression as, boosters work off vacuum created by engine suction. Poor suction=poor vacuum.
 
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