Air in brake lines, rears only
08-27-2007, 01:23 PM
#1
Melting Slicks
Thread Starter
Air in brake lines, rears only
I posted this in C4 tech then thought it would be better here.
Car is a track use only (mostly auto-x) 94. Carbotech pads with C4 13" brakes in front, stock size rears. Uses only Motul fluid.
I have a problem with air in the brake lines on the rears only. No obvious leaks around the calipers, bleeders, lines, booster or master.
I can bleed the system and it wil be fine for about two weekends, 30 to 56 passes, of auto-X. Then the brakes will get start to get mushy and eventually go to the floor. (By the way, that's not real fun after a long straight section).
I can then jack it up and pull the rear wheels off and go through the bleed process. The first one or two depressions of the brake pedal and the fluid is fine then there will be a decent size air pocket, not just a bubble, come through the line.
I can do both sides, put it back together and it's fine again for a couple of weekends. I never have air in the fronts. Any suggestions? Is this indicative of air in the ABS unit?
This has been going on for several months now and I'm getting a little tired of it.
Car is a track use only (mostly auto-x) 94. Carbotech pads with C4 13" brakes in front, stock size rears. Uses only Motul fluid.
I have a problem with air in the brake lines on the rears only. No obvious leaks around the calipers, bleeders, lines, booster or master.
I can bleed the system and it wil be fine for about two weekends, 30 to 56 passes, of auto-X. Then the brakes will get start to get mushy and eventually go to the floor. (By the way, that's not real fun after a long straight section).
I can then jack it up and pull the rear wheels off and go through the bleed process. The first one or two depressions of the brake pedal and the fluid is fine then there will be a decent size air pocket, not just a bubble, come through the line.
I can do both sides, put it back together and it's fine again for a couple of weekends. I never have air in the fronts. Any suggestions? Is this indicative of air in the ABS unit?
This has been going on for several months now and I'm getting a little tired of it.
08-27-2007, 01:33 PM
#2
Melting Slicks
My guess is either you have a bad caliper (air in from the pistons, bleed screw) or there's a poor brake line connection somewhere in the rear of the car...at either the caliper or someplace else.
How old are your calipers? Lines? It's cheap insurance to replace them if they are more than a year old. Lines last quite a while, but if they are more than 2 years old, I'd look at replacing them too.
How old are your calipers? Lines? It's cheap insurance to replace them if they are more than a year old. Lines last quite a while, but if they are more than 2 years old, I'd look at replacing them too.
08-29-2007, 09:59 PM
#3
Melting Slicks
My guess is either you have a bad caliper (air in from the pistons, bleed screw) or there's a poor brake line connection somewhere in the rear of the car...at either the caliper or someplace else.
How old are your calipers? Lines? It's cheap insurance to replace them if they are more than a year old. Lines last quite a while, but if they are more than 2 years old, I'd look at replacing them too.
How old are your calipers? Lines? It's cheap insurance to replace them if they are more than a year old. Lines last quite a while, but if they are more than 2 years old, I'd look at replacing them too.
If no oil leaks showing, might look at the master cylinder, IMO
08-30-2007, 10:09 PM
#5
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I would look at the piston seals in the rear and check rotor/bearing run out in the rear. Although your brake system is different than C2s and C3s they had a problem with air pumping into the brake caliper cylinders due to rotor run out in combination with their spring loaded pistons that made the pads stay in contact with the rotors. With enough run out the pistons would pump air right past the seals.
You could have something similar happening.
Bill
You could have something similar happening.
Bill
08-31-2007, 07:30 AM
#6
Melting Slicks
Thread Starter
Thanks for the input.
I put it up on stands and pulled the wheels. Then I crawled around under it with a drop light and there are no signs of leaks anywhere in the lines, mc, booster, or the calipers.
I bled the ABS unit on the possiblity it had air and only got two very small bubbles. Slightly larger than a pin head.
Then I bled the rears again. I got the usual large pocket out of both but I kept bleeding them. Then I got more air after several more pumps. I kept going on both rears (one at a time of course) until absolutely no air was in either line. I did the fronts too while I was at it and as usual got no air out of them.
Is it possible that the rears brakes get hot enough to boil the Motul? I always thought it was the fronts that were more likely to boil.
I put it up on stands and pulled the wheels. Then I crawled around under it with a drop light and there are no signs of leaks anywhere in the lines, mc, booster, or the calipers.
I bled the ABS unit on the possiblity it had air and only got two very small bubbles. Slightly larger than a pin head.
Then I bled the rears again. I got the usual large pocket out of both but I kept bleeding them. Then I got more air after several more pumps. I kept going on both rears (one at a time of course) until absolutely no air was in either line. I did the fronts too while I was at it and as usual got no air out of them.
Is it possible that the rears brakes get hot enough to boil the Motul? I always thought it was the fronts that were more likely to boil.
08-31-2007, 07:49 AM
#7
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How did you bleed the calipers? did you use a Pressure bleeder?
Do you bleed before each auto-X event? that is a must do. going two weekends is too much with out bleeding are replacing with fresh brake fluid will cause this problem.
The only why to completely get the air out of the ABS unit in the caliper is with 12+ psi of Tech II or a Motive power bleeder.
a mini-vac or speed bleeders will not do this.
If this does not work, might be time to look at the Master cylinder and brake boost unit.
Good Luck
Do you bleed before each auto-X event? that is a must do. going two weekends is too much with out bleeding are replacing with fresh brake fluid will cause this problem.
The only why to completely get the air out of the ABS unit in the caliper is with 12+ psi of Tech II or a Motive power bleeder.
a mini-vac or speed bleeders will not do this.
If this does not work, might be time to look at the Master cylinder and brake boost unit.
Good Luck
08-31-2007, 08:14 AM
#8
Melting Slicks
Thread Starter
I've heard of the Motive bleeder but I've never seen one. How does it work? Does it force fluid in through the MC or pull it out at the calipers?
Last edited by jwt1603; 08-31-2007 at 08:22 AM.
08-31-2007, 08:44 AM
#9
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To start get out all the old brake fluid from the MC, and replace with new brake fluid. Check fluid level on MC after each caliper is bleed.
Pressurized tank with 12-50 to 20 psi of air. attaches to the MC, and once you open the bleed screw on the caliper, it pushes the brake fluid though the MC, Lines and caliper.
After each caliper is bleed, pump the brake peddle 3 or 4 times.
Remember brake fluid is costic and will eat paint.
Pulling fluid out of the caliper aka speed bleeders or gravity bleed does not open the ABS unit in the caliper.
HTH
08-31-2007, 11:47 AM
#10
Racer
Joe
Come by the garage and I'll show you a pressure bleeder. I just bought the adapter to do late model Chevy M/Cs and am in the process of getting it hooked up to my old bleeder. I used it all the time on my '69 and it was one of our race team's 'secret weapons' back in the '80s when none of the GT1 Vettes had good brakes.
The one thing I thought about is the possibility that the fluid might be boiling near an exhaust compoent. You run headers in Group II, so it might be different than my Group I car.
The pressure bleeder is great but will go through a ton of fluid before you know it. I'm going to switch from the Motuls to the Valvoline and see if the Motuls is worth the price difference ($.95 @ oz. vs $.25 @ oz. from Pegasus).
Locke
Come by the garage and I'll show you a pressure bleeder. I just bought the adapter to do late model Chevy M/Cs and am in the process of getting it hooked up to my old bleeder. I used it all the time on my '69 and it was one of our race team's 'secret weapons' back in the '80s when none of the GT1 Vettes had good brakes.
The one thing I thought about is the possibility that the fluid might be boiling near an exhaust compoent. You run headers in Group II, so it might be different than my Group I car.
The pressure bleeder is great but will go through a ton of fluid before you know it. I'm going to switch from the Motuls to the Valvoline and see if the Motuls is worth the price difference ($.95 @ oz. vs $.25 @ oz. from Pegasus).
Locke
08-31-2007, 05:05 PM
#11
Melting Slicks
I have heard of people having trouble getting all of the air out of the C4 ABS system. Talk to somebody who really knows the system and make sure that there isn't any air in there.
If you had air in the abs it might not come out until you start cycling the system, and that would happen after a few runs, depending on how hard you were into the brakes...
As I recall (and it has been a while) that you needed to do more than just bleed at the wheels with a "pump the pedal" bleeding technique to get the pump and other stuff cleaned out.
Again, suggest you look into the manual and find somebody who knows the system and can properly bleed it...
If you had air in the abs it might not come out until you start cycling the system, and that would happen after a few runs, depending on how hard you were into the brakes...
As I recall (and it has been a while) that you needed to do more than just bleed at the wheels with a "pump the pedal" bleeding technique to get the pump and other stuff cleaned out.
Again, suggest you look into the manual and find somebody who knows the system and can properly bleed it...