Another Brake Question

[Mr. E]

I've read lots of threads on this, but...

I have a 68 that needed some brake work so I got a complete rebuild kit from Eckler's and had it installed ~ 12 years ago. Everything was fine until about 1.5 years ago when I developed a leak in one of the calipers. The shop worked with Eckler's who agreed to replace it as a defective part. Brakes were fine again until this summer when they suddenly got soft and went out one day (no pedal at all) after an extended highway drive. The shop inspected everything, found no leaks, and bled the system really well. They were rock hard and worked great until the engine got hot (long highway drive home-no brakes on the exit ramp). The shop ended up replacing the master cylinder and bled 2.5 quarts through. The guy swears there are no leaks and there was no air in the system. Again, rock hard. I test drove it for 15-20 minutes in some nearby empty parking lots and neighborhood streets and all was fine. Then...after the highway drive home, no brakes getting off the exit ramp.

I trust the guy: He's a well known chassis, suspension, brake specialist who is on several race teams and works pits for some race teams including at LeMans. He's baffled as well. I have SS sleeved calipers and use DOT 5 fluid.

Any ideas?

 

[duke94]

Most common cause for this is to much rotor runout. This makes the caliper pistons move in and out while driving and suck air past the piston lip seals. Have your mechanic check runout and shim the rotor to get it back to spec. The ultimate fix is to go with o'ring seal calipers as they will never suck in air. But, the rotor fix will do you just fine.

 

[JohnZ]

Anyone with the slightest knowledge of '65-'82 Corvette brakes would spot your symptoms instantly as air-pumping due to excessive rotor lateral runout; classic symptoms of a chronic problem. Get rotor lateral runout down to .003" or less and the problem will disappear.

 

[Mr. E]

I called the mechanic and as soon as I began to mention the rotor runout, he said he checked for runout and adjusted everything so it was dead on the first time he worked on the brakes.

While I was picking up my G35 from him last week, his brake tech was explaining the break-in I was going to get with the new perfomance pads and asked if the Vette was finally OK because he hadn't seen it in the shop for a few months. I started to explain it all to him and he also immediately mentioned runout as the usual problem. I told him Bill said he checked it the first time and said everything was perfect.

Anyway, he said he'd go through the whole process again and probably install the O-ring seals for me anyway if I want. For the price of the O-ring seals, I think I'll do that even if he rechecks and finds the runout out of spec.

 

[Hib Halverson]

There are actually two issues which are key to this air-pumping thing about 65-82 disc brakes.

First is the aforementioned runout...actually either runout or thickness variation can cause air pumping.

Secondly, in the rear, if you have too much bearing play, even if the rotors are perfect, you'll get air pumping.

That the pedal is rock hard after bleeding then goes soft as you drive is a sure sign of air pumping.

So, you need to check for rotor run out, rotor thickness variation and rear bearing clearance and remember, the clearance at the bearing is multiplied by the rotor's radius so even if you have a bearing clearance that within the GM spec (.001-.009 as I recall) you can still have enough "run-out" due to bearing play that you'll get air pumping.

For more information on servicing 65-82 disc brakes see:
http://www.idavette.net/hib/BBfHInet12.htm
http://www.idavette.net/hib/BBfHInet13.htm

 

[GTR1999]

John and Hib are correct. The o ring calipers were designed to eliminate runout and endplay issues. A lot of guys use them without a problem but the lip seal system works very well and has since 1965 when all other were still using drum brakes.

You said you guy checked and adjusted the runout, I would inquire about this. How did he check the runout, did he check the endplay, and finally how did he correct it? Chances are the rotors are no longer riveted to the hubs or spindles. Are they bolted on or just left in place by the caliper and wheel lug nuts?

I have posted many times on correcting this problem and rather then post it all over again, I can refer you to some links that will help. PM or email me and I'll get the info to you. Now if you're relying on someone else to do the job to the detail some of us do you may be in for a let down. If the runout and endplay is held under .002" lip seals, stock rotors, organic pads and good old DOT3 will be fine for many years. If you store the car or for just good maint bleed the system every 2 years. If done like this they'll throw you through the windshield and not pump air into the system.

When I was an 18 YO kid I had a 67 and my buddy a 68. Every Sat afternoon we had to bleed his brakes. We didn't know any better and really didn't care then either! Now the 67 & 68 are long gone but my vettes stop on a dime.

 

[raybies]

What the other guys said is totally correct I went through the same thing with my 1980.
I had never dealt with the ridgid mount caliper set up before I bought my vette so it took me along time to figure it out. One thing i did read though is that you said you used DOT 5 brake fluid which is the silicon based fluid and what you should be using, at least what the manufacturer and most others tell me is DOT 3 fluid which is glycol based.