Bluewasp

Can I get the basics on "runout" I had my front rotors turned and now my brakes feel a little soft when you push hard, and they squeak now.

thanks

Jvette73

Runout is the term to describe whether the rotors are true(straight) or not. A soft pedal has nothing to do with runout. With excessive runout, shaking or pulsating can sometimes be felt when applying the brakes. Soft pedal is most likely due to air in the system.

GTR1999

To check runout you'll need a dial indicator. An indicator is used in the Machine-Tool trade to measure movement in thousands on an inch(.000-.999" typically, some measure more or less).
You need to jack up the car,remove the wheels and mount the indicator by mag or clamp so it doesn't move-this is very improtant, any movement and it will throw off your measurement. Once secure then bring the indicator tip in contact with the rotor. Depending on the type of indicator you use, you will want to put a preload on it. I use a 4" indicator for this so I load it about .050" and set the dial at "0". Mark a referance point somewhere on the rotor so you know when you have gone 360*. Now slowly rotate the rotor and watch the dial. It will move to the "+" or "-" side of "0" add the total amount over 360* and that your runout. Rotors should be less then .005 total, the closer to "0" the better. The rears are checked the same. Correcting runout leads to differant approaches.
Good Luck, if you can't borrow an indicator they can be purchased for about $50.
Gary

norvalwilhelm

Gary covered it pretty good but don't forget to reinstall the lug nuts and torque them down just like you would if the wheel was on. If you find runout you can cut shims from pop cans and put them over a stud to move the rotor out on the low spot.

Gary79

to address the "soft" feel,check the fluid level in your master cylinder.you may find it neccessary to bleed the brakes. in regards to brake squeal,remove the pads and sand the "glaze" off the pads and re-install.

Tom454

When checking front rotor runout, I always adjust the wheel bearing to -0- play so that the dial indicator does not add this (wobble) into the equation. Normally, if you are careful while turning the rotor 360, you can avoid this mistake. Setting the bearing to -0- precludes it completely. The rear bearings can also have the same effect, but you cannot set the rear bearing play temporarily to -0- unless you have slip fit bearings/spindles. When I rebuild trailing arms, I use a jig to hold the spindle support in a vertical position, and take a rotor runout measurement at this time. This precludes bearing wobble as an influence on the dial reading for the rears.

I have seen "pros" mistake bearing wobble for rotor runout many times.

Just a heads up.

Tom

Turbo-Jet

another good way to shim the rotor true is to use arbor spacers over the lugs available from a machine tool supply store for under $10. get a bag of .001, .002 and .003 with 1/2" ID. these work great and are usually hardened. with a little work, i got mine down to less than .001 TIR.

Bluewasp

thanks for all your help... One thing I did forget to mention was when the pedal gets softs it feels like the rotor is warped. it shakes a lot. So, I am thinking I might have some air and the rotors have runout.

Turbo-Jet

with fixed calipers like ours, air in the fluid is a result of runout

Chuck Gongloff

Corvette rotors won't take too much turning. There's not a lot of "meat" there. Don't remember the minimum spec now, but it's embossed on the back of each rotor. Agree with all that others have said. Would also add that if the rotors have been turned too thin...under spec...they tend to get hotter and then warp more. Severe runout could produce a pulsing pedal. The more "normal" runout causes air pumping back into the calipers and progressive loss of pedal. Did a reputable shop turn your rotors? Did they just turn the rotor, or the rotor/hub assembly? Are the rotors on your car the originals, still riveted to the hub, or have they been replaced already? Let us know :) Hope this helps. Chuck

Turbo-Jet

too thin, that is exactly what was the case with my rears. the minimum thickenss is 1.215" (1.250" new). mine were 1.190", and had a runout of .015". pumped air and squeaked. simple fix.

mayberg

Where exactly does one get these various and sundry arbor spacers? I checked Harbor Freight (phone), Steve's Wholesale (phone), and my Wholesale tool catalog.

This sounds like a much better deal than the pop cans I'm trying to use.

majglc

You probably have a rotor runout problem which is causing the caliper to suck air into the system. If the rotors are thick enough to be turned, the rears can be done on the car. I had mine done at the local Western Auto.

Turbo-Jet

try Reid Tool Suppy, out of Michigan: http://www.reidtool.com/

1/2 x 3/4 x .003
PB-25124

1/2 x 3/4 x .001
PB-25121

10 pc per bag

mayberg

Cam,

Thanks bunches. $35 worth of shims are on order.
GM

flynhi

Rebuild your calipers with Zero Tolerance pistons to eliminate air in system due to runout pumping.