Air in brake system

[Rick Dorminy]

I have a '77. I am having a problem with air somehow getting into the brake system. It occurs when i have to hit the brakes pretty hard. The brake light comes on and the system has to be bleeded. There is no loss of fluid and no leaks out, only air going in.
Does anyone know where it could be coming from? I replaced one of the calipers and lines but everything else is original.

[Wayne W.]

Re: Air in brake system

Let me guess, Its always in the rear calipers? It has runnout in the rear rotors. Check the archives for more information.

[Ed Jennings]

Re: Air in brake system

I had this problem once and checked my rear rotors. Had .025+ runout on one of them! Max allowable is .003-4 or something lkie that. Turned the rotor, end of problem.

[John C. Woods]

Re: Air in brake system

Rick: Due to the manner in which the caliper seals are designed, total run out (wobble) of the rotors causes the brake calipers to actively PUMP AIR INTO THE caliper.

The maximum total runout spec is .005 inch or less. The closer you get the rotors to true (the absence of wobble) the better off you are. It's an easy check to make using a dial indicator and magnetic block/linkages. Any brake shop can do it for you. They can also true up the rotors. If they are too thin to turn, replacing them can be a fix. However, I've seen situations where the problem in the rear is actually with the axle hub, not the rotor. That can be fixed but it is expensive to do so. A machine shop can turn them true, with or without the rotors, but obviously that means that the axle needs to be out of the trailing arm, and that means new rear bearings, and all that is involved with that operation. You can avoid the machine shop bit by buying new axles but that's costly too and you don't avoid the challenges of getting the old axles out, replacing bearings and so forth.

Don't ignore the front rotors, although a less likely source of the problem. Any rotor can cause the problem.

Bottom line, the source of the problem is most likely out of tolerance total run out of one or more rotors, but the fix that might be required is not as obvious. Actually, this makes bleeding brakes from time to time look like a not so bad alternative...

Good luck!

[Rick Dorminy]

Great info, thanks folks...

I will check the rotors. I'm sure they aren't true. Didn't realize that the pistons were able to pump air into the system. I have a squeek on the left front from the brakes so that could very well be the source.
thanks again..

[John C. Woods]

Re: Great info, thanks folks...

The piston seals are designed to use the fluid pressure to assist in maintaining the seal against the piston cylinder, from the inside. When the piston is pressed into the cylinder, from the rotor pushing against the pad and no applied brake pressure, the seal does not have the advantage of the additional seal added by the internal fluid pressure. Air slips past the seals and is effectively "pumped" into the piston chamber. Good luck on finding and solving the problem!

[Jeffrey S.]

Re: Air in brake system

Rick:
The solution I used for the same problem on my 69 fixed the symptoms but not the root cause but since doing it (several years ago) I have not had any problems - the brakes are better than they ever were.
I replaced the pistons and seals with o-ring pistons and seals. The calipers were already sleeved so it was a rather simple, straight forward, and inexpensive fix. If the calipers have not been sleeved, then buy new ones from any number of vendors that advertise o-ring calipers.
As I said this does not treat the root cause but for me solved the problem completely.
Hope this helps!
Jeff

[John H.]

Re: Great info, thanks folks...

Most quality brake shops (not the discount Bubba down the street in the converted gas station) and most Chevy dealers have equipment that will re-surface rotors on the car, which is the key issue in eliminating lateral runout in Corvette rotors. Any time you remove or replace Corvette rotors it's essential to dial-indicator check lateral runout for less than .005" deviation, or risk the "air-pumping" phenomenon.

[Patrick H.]

My pet peeve on this one is...

... WHY all the monthly Corvette magazines always omit this fact is beyond me. They probably cause more braking problems with their articles than they provide solutions.

Even in the last fwew months they showed once again to remove the rotors, slap on new ones, and drive off.

I'll bet Joe L. goes nuts when he reads that.

Patrick

[Terry M.]

Re: Air in brake system *TL*

Excellent point Jeff. For those with freshly rebuilt calipers and air pumping problems due to rotor wobble - you can try to minimize the wobble by thoroughly cleaning the interface between the rotor and the axle shaft (for rear rotor problems).

Rotate the rotor relative to the axel shaft and re-measure the run-out to see if that will reduce it enough.

Failing those attempts, there are folks (I believe Stainless Steel Brakes in New York in one) who make tapered shims that fit between the rotor and the axle to reduce the run-out.

Of course John Hinkley's advise to find a shop with on-the-car rotor cutting equipment is also an option. Many modern front wheel drive cars almost require this kind of service as removing some of those front rotors takes heroic efforts. The on-the-car servicing saves mucho time and dollars.
Terry

[Wayne W.]

Re: Air in brake system

I am not too sure those things work very well on the rear of a Corvette. I have asked several technicions to explain to me how they could guarantee that there would be no runnout if the bearings were loose. I have yet to get a good answer. Think about it. A rotor mounted on a shaft that is near the loose limits and you strap on a driver to turn the rotor. The cutters may run true to the studs and the rotor, but the shaft may be wobbling like a top, which is exactly the problem that you are trying to solve. When you then let the car down and load the bearings the rotor will still wobble. No solution. These units are designed to work on rotors that ride on no clearence bearings.

[Terry M.]

Re: Air in brake system *TL*

No disagreement from me Wayne. If the bearings are loose all bets are off. With proper rear bearing clearances, however, I believe it would work, but I have no first hand experience to base that on. I simply believe it is another option to consider.

Some of the on-the-car units use the vehicle power to turn the rotors. On Corvette this requires special care, but that is not unlike some FWD vehicles.

Terry

[Wayne W.]

Re: Air in brake system

There in lies the problem. All 63-82 rear bearings are loose. They have a tolerance of .001-.008 or so. Now translate the movement allowed by a bearing set up somewhere near the center of the range out to the edge of the rotor. Nearly five inches away. The resulting runnout could be considerable. I think that, unless you know that your bearings are very close to zero, this method would be unreliable.