No Brakes when engine is started
Thread Starter
Burning Brakes
Joined: May 2010
Posts: 941
Likes: 10
No Brakes when engine is started
I have a strange brake problem with this 1979 I am bringing back to life.
I had to put to rear calipers on, plus all new brake pads. The Master Cylinder appeared to be original and rusted badly, so I replaced it with new. The master cylinder has been bench bleeded with normal good bleed results, no air. I have gravity bleeded the wheels using 3 quarts of fluid, 2 times now. I have a super normal hard pedal, as I would expect......with the engine not running.
Start the engine so the power booster gets vacuum, and the brake pedal, when pushed goes right to the floor with no resistance whatsoever. The vacuum booster and check valve hold vacuum for a long time after engine is shut down.
I never had a problem like this before, what am I missing. Opinions on whats wrong.
Thank You
8Valve
I had to put to rear calipers on, plus all new brake pads. The Master Cylinder appeared to be original and rusted badly, so I replaced it with new. The master cylinder has been bench bleeded with normal good bleed results, no air. I have gravity bleeded the wheels using 3 quarts of fluid, 2 times now. I have a super normal hard pedal, as I would expect......with the engine not running.
Start the engine so the power booster gets vacuum, and the brake pedal, when pushed goes right to the floor with no resistance whatsoever. The vacuum booster and check valve hold vacuum for a long time after engine is shut down.
I never had a problem like this before, what am I missing. Opinions on whats wrong.
Thank You
8Valve
Thread Starter
Burning Brakes
Joined: May 2010
Posts: 941
Likes: 10
Thanks for the reply, and a good thought. However, the old Master Cylinder had the same issues; ie hard pedal with engine off, goes to the floor with engine running (vacuum on booster) So its not the actuating rod, you could see from crud and rust the the MC had not been off for many years.
8Valve
8Valve
Burning Brakes
Joined: Sep 2009
Posts: 1,124
Likes: 140
From: Western New York
You have a new master cylinder but did you replace the booster? You said you had the same problem before the new MC, so I would say the power booster is the bad part in this equation. Maybe someone who knows PB will chime in here and confirm.
Duane
Duane
Thread Starter
Burning Brakes
Joined: May 2010
Posts: 941
Likes: 10
I dont understand how the power booster can be bad.
With the engine not running, I push on the brake pedal and its hard. I am a big guy and really push hard on the pedal, it does not go down . The power booster just amplifies my foot push. With the engine running, the pedal goes to the floor, it goes down with very little push.
I just read thru 10 pages resulting from the search, thats difficult on dial up at 24.4k. Others have had this problem and reported that the final fix was air in the lines, bleeding fixed it ???
8Valve
With the engine not running, I push on the brake pedal and its hard. I am a big guy and really push hard on the pedal, it does not go down . The power booster just amplifies my foot push. With the engine running, the pedal goes to the floor, it goes down with very little push.
I just read thru 10 pages resulting from the search, thats difficult on dial up at 24.4k. Others have had this problem and reported that the final fix was air in the lines, bleeding fixed it ???
8Valve
Racer
Joined: Nov 2006
Posts: 264
Likes: 0
From: McLouth Kansas
When I first got my vett some years ago my pedal was hard as a rock with the engine not running and with the engine running I had difficulty stopping the car since the pedal would almost hit the floor pan.
I replaced the booster and problem solved.
I replaced the booster and problem solved.
Burning Brakes
Joined: Jul 2015
Posts: 823
Likes: 57
From: Austin TX
According to Cardone, "The booster will never cause a low or soft brake pedal." If this is correct, you might still have air in the lines.
https://www.cardone.com/tech-help/br...ster-questions
https://www.cardone.com/tech-help/br...ster-questions
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Thread Starter
Burning Brakes
Joined: May 2010
Posts: 941
Likes: 10
According to Cardone, "The booster will never cause a low or soft brake pedal." If this is correct, you might still have air in the lines.
https://www.cardone.com/tech-help/br...ster-questions
https://www.cardone.com/tech-help/br...ster-questions
I feel confident that I found the problem today,
a miss diagnosis. I have painted the vette and put a new interior in the car. To keep the interior clean everything is covered in sheet plastic, including the carpets.
When I was trying the brakes with engine running and off, the plastic prevented me from looking directly at the brake pedal position. HA !, I pulled out the plastic sheet around the brake pedal today. With engine off, the brake pedal is (eyeball measurement) maybe 2.750"
off the floor. Start the engine and with the booster active the brake pedal goes down to within maybe (eye ball measurement 1.500" from the floor. It does not bottom on the floor !
So, in my opinion, the system is operating properly.....except.....that it still has captive air somewhere. With that said, I ordered a motive pressure bleeder today. It will be here on friday. I feel confident that once I pressure bleed the entire system, all will be well.
With the plastic hiding the actual view of the brake pedal, it sure felt like it bottomed out on the floor when the booster was active (engine running)..
8valve
Last edited by 8valve; Mar 16, 2016 at 07:22 PM.