Comp Cam 1

Took my C2 out for probably the last ride of the season today. When I hit the brakes coming off the highway the brakes felt like they were pulsing back as I applied the brakes. What gives? Side note I do have disc brakes all around. Thanks for any help!

62Jeff

Warped rotor(s)

MikeM

Don't know if you have disc brakes or not but I've never seen a warped rotor on a mid year Corvette.

Comp Cam 1

Thanks guys,, I do have discs on the car.

nassau66427

ABS engaging?

Or maybe too much runout on a/some rotor(s)?

magicv8

rear drums on the car? egged out? bad rear spindle bearings? hot spotted?

Dan Hampton

Do you feel the pulsating through the steering wheel?

kellsdad

As was said before, pulsations felt in the brake pedal when braking is usually due to one or more warped brake rotors. It typically takes a lot of heat to warp a rotor and heavy duty rotors that were common on 65 and up Corvettes were engineered to take a lot of abuse. If you've been using the brakes heavily (e.g., track use or extended braking down a long steep hill), or it you have aftermarket rotors, that would be the first thing I'd check.

MikeM

As I said before, never seen a warped Corvette brake rotor. I have seen brake pad material stuck to the rotor.

This can happen when you use the brakes hard enough to get them hot, then stop and hold your foot on the brake and there is material transfer from the pad to the rotor. If you look at the rotor for this condition, you'll probably see no more than a shadow on the rotor. You can get it off with sandpaper.

Brake shops like to call it "warped rotors" 'cause they charge big bucks to cut your rotors or install new ones.

Bill32

Yea, really.

Warped rotors are rare and everyone calls what Mike described "warped".

If you want to check your rotors, buy a cheapo dial indicator from HF for $10, C-clamp it to a jackstand and check the rotors.

Brake pads (when bedded in properly) run on an adherent layer of pad material transferred to the rotor. Pads that aren't bedded in properly or are under light use are functioning abrasively, the pads actually scrape off the adherent layer of material on the discs. Then what happened is what Mike said, hard stop (at a traffic light) and the brakes being held on while hot. This deposits some of the material on the rotor and creates the "warped" feeling.
Do what was said and look for the color change, sand it off.
I use the 2 inch scotchbrite "rotolok" pads on the entire disc. You can get these at HF and use a drill or r/a air grinder (the HF rubber holder actually works better that the 3M, doesn't crack), $13.

Then rebed the pads, new or used. Bedding will put a nice even adherent layer of material on the discs.

You can tell the difference between a warped rotor and this material transfer by:
If you have a "pulsing" coming up to a stop, light pedal, it's probably material transfer.
If you do a hard stop at at least 60 and the vibration is immediate and heavy, it may be the rotor.
The material transfer thing is way less noticeable at higher speed and harder pedal stops.

Mike Ward

MikeM is correct about warped rotors being an overblown myth, especially on C2/C3 Corvettes, but I'm not sure I agree with your method of telling the difference.

Seems to me that the pedal pulses at all speeds under all braking conditions.

csysman

Would bet your rotors need to be turned

Bill32

Could be my description, pulsing at all speeds/conditions may be the rotor.
Pad transfer on one section of the rotor (as Mike described) will be almost unoticeable ulder hard braking.

Again, the only way to do it right is to put a dial on the rotors.

Mike Ward

My experience differs, material transfer caused pulsation at all speeds/conditions, no warpage. Root cause was poor quality jobber pads.

toddalin

Sounds like excessive runout on the rotors to me. Get this checked and fixed before you loose your brakes on the freeway ('cause that's when it will happen) like I did when this happened to me (and I'm not the only one here who this has happened to).

MikeM

Please tell me more. What is excessive and how did you lose your brakes?


Another way you can tell is if the severity of the sensation of brake pulsing is a variable from one day/week to the next. If it is, that's pad material transfer.

PS. Some of these brake shops will even charge you to turn new rotors right out of the box, claiming even new ones have runout.

Tampa Jerry

If your rear rotor rivets have been drilled off and your lugnuts are not tight (80lbs.) you could also feel a pulse when using the brakes. You could also get a pulse through a bent spindle shaft. It could happen if the rear wheel slides into a curb. If the lugs are tight, put a dial indicator on the rotor. You could even switch the rotors around to see if the problem is the spindle shaft or the rotor itself. Just mark everything before you start so it can go back without trouble. Jerry

Bill32

That's easily possible,Mike. By no means did I mean that transfer couldn't be felt at all speeds, too many variables to have one diagnosis fit.

I have done a couple of C4's lately (including my own) where the pulsing could only be felt below 20mph.
My Silverado's the same way, feels smooth until you're below 20.

Yea, the jobber pads and poor quality.
And some pads don't like to be mixed with the old layer on the rotors. That's why I always clean up the rotors for new pads.
We've really experienced this problem trying to mix PFC and Hawk pads at the racetrack.

toddalin

Excessive runout is caused due to warpage of the rotor surface. It creates high and low spots that the pads follow. This then pulses the pads in and out. I couldn't actually tell you in mm what constitutes "excessive."

As the pads move in and out they can suck air in through the seals letting air into the system. On the freeway this can go on for mile after mile until you've sucked in enough air such that the system becomes "compressable" and when you push your pedal..., nada.

Happened to me on the off ramp and I was lucky to get the car stopped. I pulled into the 7-11 at the end of the ramp and had to have the car flat-bedded home. (A GREAT reason to retain the rear drums is the full sized emergency brake).

MikeM

Poppycock! Old wive's tale.

A couple of revolutions of rotor runout and you'll push all four pistons back the amount of the runout on both sides of the caliper halves and the fluid it takes to hold them against the rotor, back into the master cylinder reservoir. The springs behind the pistons aren't strong enough to make the caliper pistons follow runout in the rotor at any road speed at all.

If you've got air in the system, it's because you had bad/defective caliper seals.

Material transfer from the pads to the rotors that can cause the sensation of brakes pulsing can be as thin as .001" or less. That's less than the allowable runout on brake rotors.