I have a really weird brake problem. I replaced all 4 calipers, and pads. I also upgraded to stainless steel braided brake lines. I bled the entire system. I had great brakes for about a week. The brakes got worse and worse, but I was able to pump them up. I then replaced the master cylynder and re-bled everything. The brakes were great for a week. I then replaced the replacement master cylynder and re-bled everything. It worked great for a week, then back to no brakes. There are no fluid leaks in any of the calipers, and when I bleed the system, it seems like there is only air in the front brake lines. Any suggestions? Thanks!

 

I'm guessing you didn't do anything with your rotors?

What it sounds like is you have excessive runout, or wobble, on one or more of your rotors. If a rotor is wobbling, the pistons in that caliper are moving in and out very fast, and act like an air pump sucking air in. They usually won't let fluid out...just air in.

It sounds like one or more of your calipers are sucking in air...giving you soft brakes.

I would suggest pulling all four of your calipers and having them tested for runout or turned on a lathe. You need them very true. Oreilly can usually do this, as well as ANY local brake shop.

 

Start with the rears, they're usually suspect and known to be the cause of this problem.

 

Good points on the runout also check bearing endplay. You want them both under .002" .003 is ok but I wouldn't try it over that.
The concern with turning these rotors is they are not going to be concentric when done. They may look great but the center line of the spindles is not going to be perpendicular to the cut. Relaying on lug nut to hold the rotors true will also change every time the wheel is removed.
Until you get these 2 corrected, if out, you'll have this problem.

 

GTR1999,

OK, I'll bite. How does a machine shop turn the rotors so that they will not be perpendicular to the axle line? How is their machine setup deficient?

If that is true, then I can not separate a rotor from an axle and ever expect it to run true enough to keep out of brake trouble? The rotor must be turned while mounted on the axle. So I have to pull the axles whenever I want to have rotors turned.

What about replacing rotors. Do you have to take a new rotor, rivet it to the axle and then have the new rotor turned?

I need an education, please help.

Bob

 

think you got a bad caliper

 

Older rotors get a lot of rust scale buildup in the area where the rotor contacts the hub also. If that area isn't very clean and flat there is no way you will achieve the runout specs. If your rotors are still riveted on from the factory the rust buildup shouldn't be a factor but if they're not replacements aren't that expensive. You can cut up some beer cans for shims behind the rotors. Good excuse for some

Rick B.

 

There is nothing weird about it. Its about normal after most brake jobs. These things are assembled and turned as a unit, thus the riveted units. Hey, there was a reason it was done that way. When you seperate them, and turn them on a brake lathe, most of the time they will come out with runnout. Its a fact.

 

I was told it was because during production the car was put on it's sides and they riveted the rotors so they wouldn't fall off while the car was being flipped.

 

that's at least the second time you've said that. That sounds just plain dumb.

When do they put the car on its side and why? The caliper holds the rotor in place.

Who told you???

 

I don't recall the member's name. He claimed to have worked for GM on the lines where the Corvettes were made, and said that's why. I think it's a funny mental image...that's why I remember it I guess. I have no idea it's true...but I would guess a guy that worked there would.

 

The rotor were riveted to the hubs and rear spindle at the factory or from the vendor probably. This was done a very good reason, to keep rotor runout at or very near -0-. New 65-82 corvettes had excellant brakes, no ss sleeve and no problems.

When the cars sit for months.the BF aborbs moisture & turns the BF very acidic which pitts the bores. Sometimes the BF looks like mud and even with SS sleeves, the pistons can become "stuck in the mud"! I believe SSBC was one of the first to sleeve calipers back in the mid 70's. I know bought some around '78?

The rotors, in most cases, do not need to be turned. Corvette brakes get a bad name as a result of people not trained in them working on them. It was and may still be very common for a mechanic to turn the rotors. It was part of the job and they made a few bucks on it too. Most times this led to more parts be replaced then needed and the customer ending up paying well over a $1,000 for a brake job and system that will continue to pump air.

Turning the front rotor and hubs off the bearings races can be done.
Turning a rear rotor that is still riveted can be done with an on car lathe. Turning a rotor in a lathe chucked up on the rotor ID will produce a good finish but it will not be concentric with the spindle and will runout, most time worse then before. New rotors have runout as well.
I do not rivet them but instead bolt them on. I have posted about this many times and a few guys here have successfully done this. The rotors could be bolted on and turned but you give up thickness doing that, I shim them instead and can get them under .002" total.

You can go with o rings and they may help but as I've said for years I'd rather correct the problem instead of using a band aid approach.

Bearing endplay over .003 will also contribute to the problem.

Learn how to do them yourself. Save a lot on labor and over charges and set up the system like it should be. Manual and PB vettes do stop very well in stock configuration.

 

The rotors were riveted on for ease of assembly. IIRC, the factory service manuals says as much, and also says not to bother re-riveting the rotors when replacing.

If you want your rotors to be true without shimming, get the in place and take the car to a good shop with an on-car brake lathe.

 

Thanks to all. The left front rotor is brand new. It included the hub and rotor all rivited together. The right front is the original, still rivited to the hub, and was turned by a great machine shop. The rears are also originals, rivited to the hub. (The car only has 72k easy miles)They were not turned. The air always seems to be in the right front. I'll double check wheel bearing play and see if that helps. Thanks again!

 

A brake that is bled out, will not pick up air for no reason. There must be something wrong on that rotor to cause it to pulsate. Now a couple of other things. Can someone try to convince me that you can turn the rotor on a Corvette with an on car machine and guarantee that it will come out true. It looks rather impossible with the runnout that can be present in the bearings. I can see it on modern stuff, but one with a possible .008 clearance, I dont see it being too accurate. Second: The funny thing about all of this is that the GM shop manual specifies not to turn the front, but indicates that you might be able to do the rears. I never understood that one.