C3 brakes sink when engine is started
Thread Starter
Cruising
Joined: May 2011
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C3 brakes sink when engine is started
I am restoring a 70 roadster, The complete brake system is new, Willwood 4 piston calipers, Tuff stuff booster/master cyl. GM proportioning valve, GM rotors, stainless lines, braided stainless hoses.. When you start the engine the brakes sink almost to the floor. Booster/calipers/master all correctly sized for each other. Car stops fine, just an uncomfortably low pedal. Brakes have been gravity bled, power bled, reverse bled and conventionally pedal bled. Master has been bench several times. The pedal stands tall and solid when not running after sinking it will not pump up. Booster pin to master cly. clearance is to spec. Pedal rod adjustment has no affect on sinking. Vacuum is 18"+.Master cylinder and booster have been replaced, no change. This was originally a power brake car all pins brackets are to spec. I consulted Willwood and Tuff Stuff all they would say is "Its not my equipment:"
Any good ideas greatly appreciated.
efloww
Any good ideas greatly appreciated.
efloww
Is my vette stock?? HAHA
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especially if it doesnt come up all the way....I had that issue with my booster
Race Director
Joined: Apr 2011
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From: North of Toronto - Ontario
Only 4 reasons for the peddle to go down
Loosing fluid, which would be obvious
Bad Master, leaking internally, should do it regardless if the car is running or not,
Bad Booster,
Lines are ballooning up under pressure, might not be as much pressure with the car not running but if you've replaced the 4 flex lines with braided then that shouldn't be an issue
Disconnect the vacuum to the booster (and plug it of course) and see if it stops it from sinking
M
Loosing fluid, which would be obvious
Bad Master, leaking internally, should do it regardless if the car is running or not,
Bad Booster,
Lines are ballooning up under pressure, might not be as much pressure with the car not running but if you've replaced the 4 flex lines with braided then that shouldn't be an issue
Disconnect the vacuum to the booster (and plug it of course) and see if it stops it from sinking
M
Race Director
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From: North of Toronto - Ontario
When you drive with the booster disconnected, how far down do you have to press the pedal to get the brakes to come on, really come on like you are stopping regular?
The pedal distance shouldn't change, only the effort needed to get it to go that far
When you hook up the vacuum is the pedal dropping farther than it needed to be pressed to get the brakes to work without vacuum? If it is then where is that extra fluid going...
M
The pedal distance shouldn't change, only the effort needed to get it to go that far
When you hook up the vacuum is the pedal dropping farther than it needed to be pressed to get the brakes to work without vacuum? If it is then where is that extra fluid going...
M
Thread Starter
Cruising
Joined: May 2011
Posts: 11
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Without vacuum it has about 1" of toe play and is hard. With vacuum it sinks about 2" (actually passes the throttle pedal) and is soft, will not pump up. Car stops well with the pedal a little more than 1" above the floor.
Don't have a clue where the fluid is going.
Don't have a clue where the fluid is going.
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Burning Brakes
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I think a little "sink" when started is normal. 2 inches is a little excessive but it sounds like all component are high quality. How thick is your carpet?
Racer
Joined: Oct 2018
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From: England
Sounds to me like you've still got some air in the system. I had the same symptoms and yet more bleeding finally fixed it. Like you I started with an empty system and it took several pints of fluid (and several days) to get all the air out.
Is my vette stock?? HAHA
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Generally you should get very little drop with power brakes with the engine off and with your foot on the pedal and when you start it you should get an additional inch of drop if you left your foot on the pedal the whole time.
You may just need to keep bleeding it as suggested above
You may just need to keep bleeding it as suggested above
Had a 1976 L-82, 4-sp
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My '76 was like that. If you can lock up all 4's, I don't see an issue with the pedal 1" from the floor fully depressed.
Race Director
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M
Race Director
Joined: Apr 2011
Posts: 11,183
Likes: 3,335
From: North of Toronto - Ontario
Sorry, forgot you had wilwood calipers (They do outsides first then inside)
I also didn't realize they still recommended removing the calipers, I haven't played with a set of those in years.)
M
I also didn't realize they still recommended removing the calipers, I haven't played with a set of those in years.)
M
Melting Slicks
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From: Detroit MI
normal normal normal normal
If there's an adjustable pushrod between the master and booster you might gain some pedal height by taking out some clearance, but you run the risk of the brakes applying with heat if you go too far with the adjustment.
Also, depending on the pads, the car may stop very well with very little pedal effort. The feel sucks, but it probably stops well.
If there's an adjustable pushrod between the master and booster you might gain some pedal height by taking out some clearance, but you run the risk of the brakes applying with heat if you go too far with the adjustment.
Also, depending on the pads, the car may stop very well with very little pedal effort. The feel sucks, but it probably stops well.
Last edited by 69autoXr; Aug 25, 2022 at 09:53 AM.