1968 Brake Blues

[Paul P.]

I've been having consistent problems with the brakes on my 1968 convertible. Basically the brakes will be bled and then work fine. Then over the course of two or three months and about 1,000 miles, air seems to get in the system, the pedal starts to bottom out and the "BRAKE" light on the speedometer comes on when the the brake pedal is pushed hard. I can pump the pedal and it returns to normal but clearly I shouldn't be driving the car like that.

The master cylinder and all four calipers have been replaced; the rears were replaced twice. Each time it's the same story. The car does not have the J50 power brakes option. Stainless Steel brake hoses were installed by the previous owner. The car does not have a lot of miles on it, I would say under 100,000 and was until a few years ago in storage for much of it's life.

I am now wondering what my next move should be. New brake lines? Replace the stainless steel hoses? Also, is there a way to isolate the problem to the front or rear brakes?

Some Googling resulted in a article at http://www.mamotorworks.com/corvette...114-10677.html which mentioned that rotor run-out can cause these symptoms. Opinions?

Thanks in advance for any help.

[Michael W.]

Re: 1968 Brake Blues

Sounds like you have excessive runout on one or more rotors. Enthusiastic owners frequently feel the need to replace rotors on these cars without realizing the secondary problems this induces.

Remove each of the wheels to see if the original rotor retaining rivets are still in place. If not, that assembly is suspect. Secure the rotor to the hub temporarily using the lugnuts installed 'backwards' and measure the runout with a dial indicator. Remember to measure and compensate for bearing play. If you have .008" or more play and runout, this will need to be corrected.

[Dick W.]

Re: 1968 Brake Blues

Sounds like you have excessive runout on one or more rotors. Enthusiastic owners frequently feel the need to replace rotors on these cars without realizing the secondary problems this induces.

Remove each of the wheels to see if the original rotor retaining rivets are still in place. If not, that assembly is suspect. Secure the rotor to the hub temporarily using the lugnuts installed 'backwards' and measure the runout with a dial indicator. Remember to measure and compensate for bearing play. If you have .008" or more play and runout, this will need to be corrected.

Sometimes you can correct the runout by removing the rotor, cleaning the rust on the mating surfaces and reinstalling, rotating one wheel stud position. Check your runout again and remember there are five possible positions. If that does not work, you will have to have your rotors cut to true them

[Paul P.]

Re: 1968 Brake Blues

Michael and Dick:

Thank you for your responses. I'll look into the runout problem and will post what I have found.

[Michael W.]

Re: 1968 Brake Blues

Sometimes you can correct the runout by removing the rotor, cleaning the rust on the mating surfaces and reinstalling, rotating one wheel stud position. Check your runout again and remember there are five possible positions. If that does not work, you will have to have your rotors cut to true them

Sorry to disagree Dick but cutting the rotors separately from the hub is where the problems start. The rotor/hub need to be finish machined together as an assembly.

[Paul P.]

Re: 1968 Brake Blues

My mechanic was doubtful that there was an out of round condition causing the problem since we didn't feel any pulsing on the brake pedal. While listening closely however we did hear some pulsing. He removed the rotor and found it to be .080 inch off and cut the rotor (right rear, FWIW) so it is now true. It's been over 1,000 miles and the problem has not reappeared, so we are all set.

Thanks again to all who replied.






Regards,




Paul Pollock
WebCars! Webmaster
http://web-cars.com/

[Terry B.]

Re: 1968 Brake Blues

Paul, I'm curious to what type of brake caliper you are using, lip seal (what the car had originally with the modern addition of a stainless steel sleeve) or the o-ring. I copied some of the literature from a caliper vendor: Why go with O-Rings?
Lip seal design seals are used on 1965-1982 Corvette calipers. Lip seals have inherent problems. When they sit for a period of time they leak brake fluid. When they are used on a Corvette with out of spec rotors, spindles, loose bearings or a Corvette that is driven hard; they can actually suck air into the caliper fluid area causing a loss of brake pedal. This problem is called (by GM engineers) reverse air oscillation. Because of these problems, GM quit using lip seals in the 1982. O-Ring seals virtually eliminate all of these lip seal problems. Is this information correct? Terry

[Paul P.]

Re: 1968 Brake Blues

I cannot tell you what brake caliper I have other than they are not original from 1968. The mechanics (two have worked on the car) bought the generic calipers from their supplier. When I got the car two years ago the rear calipers had been replaced by the PO and they, along with the fronts, were subsequently replaced by my mechanics. From what you say, it seems that I have the o-ring type?


~paul

[Edward J.]

Re: 1968 Brake Blues

Paul. I can certainly tell you that if the problem is air and it happens as you explain it a rotor issue, As other posters have said here. And its likely one of the rear Rotors, you may not fell a pulsation if its on the rear. and even if the rotors are new this does not mean much as a lot of garbage stuff made over seas. I would do as Dick says and check run out and clean the hub surfaces sanding works cleaning the hubs. and if your using original or just old rotors the must be cleaned on the mating surfaces and then re-machined.