Sounds like air in the system. Try rebleeding but not the dot 5.
Low Brake Pedal
Thread Starter
4th Gear
Joined: Jun 2003
Posts: 4
Likes: 0
From: Whitehouse Station NJ
Low Brake Pedal
I have a 68 Vette 427/4 speed.
I have replaced the Master Brake Cylinder twice, checked the calipers and rotors and pads for worpage and wear. I have no leaks but I continue to loose my brake pedal.
My mechanic thinks that the brake fluid might be boiling and causing air to get into the system. He wants to flush the system and replace the fluid with DOT 5.
Can anyone give me some suggestions as to what my problem is?
Thanks,
Rusty Bolt from NJ
I have replaced the Master Brake Cylinder twice, checked the calipers and rotors and pads for worpage and wear. I have no leaks but I continue to loose my brake pedal.
My mechanic thinks that the brake fluid might be boiling and causing air to get into the system. He wants to flush the system and replace the fluid with DOT 5.
Can anyone give me some suggestions as to what my problem is?
Thanks,
Rusty Bolt from NJ
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
I've gone through this. To fix it, I spent half a Saturday doing a slow gravity bleed on each wheel.
If your master is new, and no leaks, it almost has to be air in the lines.
Although, I have heard of folks adjusting the pushrod that goes between the booster and master for a better pedal.
Let us know what transpires.
If your master is new, and no leaks, it almost has to be air in the lines.
Although, I have heard of folks adjusting the pushrod that goes between the booster and master for a better pedal.
Let us know what transpires.
Last edited by 73, Dark Blue 454; Jan 11, 2007 at 09:05 PM.
Instructor
Joined: Jan 2007
Posts: 195
Likes: 2
From: Wilmington MA
From my stock car racing experience, yes boiling the brake fluid can make for a soft pedal. Symtoms for boiling: pedal starts out high and hard, as you drive and you start heating the fluid in the caliper or brake line, it boils and produces a gas which makes the brakes soft and low. As the fluid cools, the now vapor turns back to a liquid and allows the pedal to be high and hard again.
If your starting w a soft pedal when cool and stays soft, chances are its not boiling the fluid. With stock exhaust and factory run brake lines, all components should be away from any heat source. A sticky and dragging caliper could cause boiling.
Are you sure you have the right master cyl? one too small would require more pedal trravel.
Look for a place your system coud be sucking in air. Not all spots where this could happen, necessarily would show a leak. Sounds impossible, but true.
When you pull up to a stop light, press the pedal twice, if first is low and the second high, then the pads are being pushed back in the calipers some how or being sucked back due to faulty or wrong master not holding a residual pressure to keep them close.
see what you find!
Frank
If your starting w a soft pedal when cool and stays soft, chances are its not boiling the fluid. With stock exhaust and factory run brake lines, all components should be away from any heat source. A sticky and dragging caliper could cause boiling.
Are you sure you have the right master cyl? one too small would require more pedal trravel.
Look for a place your system coud be sucking in air. Not all spots where this could happen, necessarily would show a leak. Sounds impossible, but true.
When you pull up to a stop light, press the pedal twice, if first is low and the second high, then the pads are being pushed back in the calipers some how or being sucked back due to faulty or wrong master not holding a residual pressure to keep them close.
see what you find!
Frank
Thread Starter
4th Gear
Joined: Jun 2003
Posts: 4
Likes: 0
From: Whitehouse Station NJ
Low Brake Pedal
Thanks to All,
The system was gravity bled I began to have this problem with the original master brake cylinder replaced it with a chrome one and them replaced it with a stock master brake cylinder.
Break Pedal is low from a cold start not that I have air in it. I can pump it up but only last for the stop and then is low again.
Why shouldn't I use Dot 5 fluid?
Rusty Bolt
The system was gravity bled I began to have this problem with the original master brake cylinder replaced it with a chrome one and them replaced it with a stock master brake cylinder.
Break Pedal is low from a cold start not that I have air in it. I can pump it up but only last for the stop and then is low again.
Why shouldn't I use Dot 5 fluid?
Rusty Bolt
Race Director
Joined: Sep 2001
Posts: 10,560
Likes: 439
From: Wellington, FL
St. Jude Donor '11 thru '25
Thanks to All,
The system was gravity bled I began to have this problem with the original master brake cylinder replaced it with a chrome one and them replaced it with a stock master brake cylinder.
Break Pedal is low from a cold start not that I have air in it. I can pump it up but only last for the stop and then is low again.
Why shouldn't I use Dot 5 fluid?
Rusty Bolt
The system was gravity bled I began to have this problem with the original master brake cylinder replaced it with a chrome one and them replaced it with a stock master brake cylinder.
Break Pedal is low from a cold start not that I have air in it. I can pump it up but only last for the stop and then is low again.
Why shouldn't I use Dot 5 fluid?
Rusty Bolt
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
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Brett Foote
Former Vendor
Joined: Nov 2005
Posts: 683
Likes: 1
From: Mounds View Minnesota
If you have pedal issues with DOT 3 DOT 5 won't help!
If you want to run DOT 5, go ahead, it works fine ( unless you race )
I think you still have air in the system.
A bad MC will pump up but then slowly drop if you keep pressure on it.
Air will pump up but be sqishey and not drop.
Vette calipers ( and all multipiston calipers ) are very hard to bleed.
If you want to run DOT 5, go ahead, it works fine ( unless you race )
I think you still have air in the system.
A bad MC will pump up but then slowly drop if you keep pressure on it.
Air will pump up but be sqishey and not drop.
Vette calipers ( and all multipiston calipers ) are very hard to bleed.
Le Mans Master
Joined: Mar 2002
Posts: 6,217
Likes: 1
From: End of the Ike IL
Cruise-In V Veteran
Cruise-In VI Veteran
For about $60 you can get a Motive pressure bleeder. It is great at forcing air out of the system from the MC. Also, it takes a lot less time than gravity bleeding.
Melting Slicks
Joined: Nov 2000
Posts: 2,738
Likes: 53
From: Bogart GA
If you have some tools and an air pump and regulator, all you need to do is put an air quick disconnect (home Depot) on to a steel plate big enough to cover the master cylinder, even an old MC cover would work. Put about 5 to 10 psi of pressure on the MC and then open the bleeders. Be sure the MC fluid doesn't run out. I do the front then turn the plate 180 deg and do the rear. Works like a charm.
Safety Car
Joined: Aug 2001
Posts: 4,883
Likes: 11
From: North Easton Mass
Did you by any chance replace the rotors? If so you have to make sure that the runout is less than a few thousandths. The springs behind the pistons cause the pistons to push the pads very slightly against the rotors. If the rotors wobble too much while turning the pads will pump the pistons in and out and allow air to get sucked in without fluid leaking out. IIRC, Zora Duntov designed the braking system and it works fine for racing and for the street IF the rotors are aligned correctly. Rust buildup on the hubs, etc. that is not cleaned off when replacing rotors will cause excessive runout and mysterious loss of pedal pressure. There is also a recommended bleeding procedure that has been debated several times on the forum. I tried what is supposed to be the factory sequence with good results:
LR Outer
LR Inner
RR Outer
RR Inner
LF
RF
Rick B.
LR Outer
LR Inner
RR Outer
RR Inner
LF
RF
Rick B.
