What would cause air to enter the brake system. There is no evidence of fluid loss. However, the pedal goes down to the floor and the brake light comes on. Calipers are fairly new and so are the pads. I've bleed system twice. Confused. Any help would help. L8TER, Paul.

 

I bought a set of calipers from Vette Brakes a couple years ago, one of the bleeder screws was put in wrong and the threads were stripped which caused air to be sucked back into the lines while bleeding them (couldn't see it until we took the bleeder out). Another had the bleeder screw put in so tight it broke off....so much for quality assurance :rolleyes:

 

Hi paul,
1-- Sounds like you have the "classic" Corvette problem. This assumes you get good brakes after a bleed and then after driving, you loose the brakes again. If this is true, then read on.

2-- If you cannot get a good pedal after bleeding, replace the master cylinder.

First I should say is that Corvette brakes (c2 &c3) are great and will provide many years of trouble free use.

HOWEVER, there are two things they will not tolerate. (Really one - with two causes).
And that is rotor runout.
We have discussed this many times on this forum, and many members have experienced this.
"What is rotor runout".
It's when the wheel turns and the rotor, for a lack of a better word, wobbles.
In other words "bent", or not running "true".

CAUSE, warped rotor, or bent hub.

RESULT, the wobble in the rotor hits the caliper pistons and pulses them in/out and pumps air into the brake system.
Result, no/poor brakes, pedal to the floor.

ANOTHER CAUSE, is when there is too much "play" in the wheel bearings and causes the same wobble and the same problem.

THE FIX, Use a dial guage to check each rotor to discover the culprit.

THE CURE, If it's only the rotor, replace it. However it could be the hub under the rotor that is the problem.
If so, shim the hub to bring the rotor to within a .001 - .002 runout.

EXPENSIVE COVERUP CURE, Use caliper pistons that use an "O" ring rubber on the piston. They are more tolerant to the runout.

Don't forget to check the wheel bearings for excessive play.

Hope this helps.
Barry

 

Barry, Thanks for the useful info. The front rotors come off the conventional way of removing the wheel bearing, then removing the rotor. Does the rear come off the same way?

 

Rotor run-out really rears it's ugly head on long hwy driving more-so-than short city driving, in stop and go traffic. This sometimes seems confusing to someone who just bled the brakes and takes for a short test drive and all seems well. Then the next day or two, goes on a serious cruise and the brake light comes on. :crazy:

Been there done that. :yesnod:

Rick :cool:

 

The rear rotors (if original) are riveted on. You need to drill the tops off the rivets. No need to drill the entire rivet. Leave the "stump" of the rivet in the spindle flange, it won't go anywhere.

IMPORTANT, mark the orientation of the rotor to the spindle. Also mark which one is left side and right side.

REASON, the rotors and rear spindles are a "matched" set. If you install new rotors on the rear, you will most certainly have a runout problem.
The original rotors were machined when riveted to the spindles.

Most spindle flanges do not run true. You could have a perfect rotor and by putting this on an untrue flange, results in a wobble.

Easy remedy is to place a shim between the flange and the rotor to bring it back true again. (Requires some trial and error)

No need to rivet the rotor back on again. It was only riveted to assist in the assembly process at the plant.

Barry

 

Thanks again Barry, If need be, might it be better to buy the rotor/spindle matched set again to reduce problems?

 

Hi Paul,
Changing the rear spindle is no easy task. You need a setup tool, box of bearing shims, dial guage, and a press to remove the old spindle and bearings.

Adding a shim between the rotor and spindle is a much easier task.
Barry

 

There are a lot of places for air to enter the system. At the line connections, at the piston seals, at the caliper half seals, at the bleeders, etc. You can find out where the air is coming from by using the brake pedal to bleed the system with a clear hose attached to the bleeder when you bleed each bleeder (the rear has 2 bleeders you know). If there is air getting into the system then you will see the bubbles come out and know that it's from that line. I discovered a leaky caliper by watching a continuous stream of small bubbles come out of a bleeder...replaced the caliper and all is good.

Also, just because it's new or rebuilt does not mean it's not going to leak. I have never seen the "rotor runnout" problem in any of the vehicles I service. Heck, my Pinto rotors are so bad I can feel them wobble in the pedal, and the floating calipers are soo loose that they vibrate horribly during panic stops. But, the system is solid and I have not had any air get into the system since I bled it in Feb 2000 when my m/c failed.

I do acknowled that the vette pedal seems to move farther and feel a bit more spongy than any other vehicle I've serviced. Just keep analysing the problem and inspect your fluid as it comes out of the bleeder to find which line the leak is in.

 

Thanks again Barry!!

Rockn-Roll,
Yes, air may enter in many places. As you said...bleed the line with a clear hose and see where air is present. OK, I've done that , however i'm seeing air bubbles in ALL caliper bleeders. There does seem to be more in the driver's rear. I also replaced the master yesterday with no positive results. As stated before, there is no leakage of fluid. I think my next step is to check front rotors. Thanks, L8TER, Paul.