Hi Sharks,

When I apply moderate to heavy brake pedal pressure, I hear a womp, womp, womp, etc. noise. It only happens when I apply the brakes. Light pressure does not result in the noise. It sounds like it is coming from the right rear.

The calipers are on tight. The rotors have normal wear patterns on the outside. Springs appear to be holding the fairly new pads in proper alignment.

With the rear wheels off, and the car in park, I can rotate either brake rotor back and forth about 1/32 of a turn. I see the CV boot turn when the rotor turns, but not the rear axle. Is a little play normal? Could this be one or more worn CV joint?

Could this be sticking brake pistons?

Could the Anti-lock Brake System be failing?

How can I diagnose the problem?

Any help would be appreciated.
<img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

 

Update:

I pulled the fuse and relay for the ABS. No change.
<img src="graemlins/c.gif" border="0" alt="[ouch]" />

 

One possibility is a "hard spot" in the rear rotor. This is a spot that has hardened slightly differently from the rest of the rotor, and has worn down a little less.

Try jacking the rear and turning the wheel by hand. If there is no periodic resistance, try having someone hold the brakes on very gently as you turn the wheel by hand. If there is periodic resistance, the answer is probably to have both the rear rotors turned or ground.

 

Thanks Wally,

That would be good news.

 

Wally,

Thanks again for cutting to the chase - I am pretty sure. I pulled the rotor, and the hidden side has the high spots. Since it is too thin to turn, Jeannie has my order. <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" />

 

Heh Wally,

Any idea why all 4 of my rotors had corrosion marks on the inner surfaces, but not the outer surfaces? The car did sit for several months after a timing belt slip. The corrosion marks looked suspiciously like brake pad footprints. Could moisture have been trapped there? 3 rotors were missing their hold down Phillips screws. One of the front rotors had been installed with screw studs still intact and slightly protruding, and rotor not oriented properly. Could this have caused warping? The other front rotor still had screws installed. I will check the orientation of the fourth rotor relative to screw studs later. I power wash the rotors often, and the spray only hits the outer surfaces. Could this account for outer surface condition?

The verdict is sweet success. I replaced the rear rotors with Porsche parts for $85 each plus tax and shipping. 928 Specialists had the best prices once again on parts. I am wondering if I should have gone with their drilled rotors… maybe when the front ones must go. A shop replaced the front rotors about 2 years ago. The corrosion was light on the front rotors, and they were barely too thin to turn. So, I buffed out the dark spots (rust) in those pad footprints, and at the ridges made by the edges of the pads. I used a wire wheel to buff the rotors. No more noise!

By the way… I was able to break the Phillips screws loose without an impact wrench. I wedged a long screwdriver between 2 studs, rotated the rotor until the screwdriver wedged against the ground. I used a ¼” ratchet driven Phillips head to free the screws.

HTH,
<img src="graemlins/roflmao.gif" border="0" alt="[hiha]" />

 

Those little phillips screws can be a PITA. I had some trouble removing mine, and on re-assembly used antisieze. Intresting though, the way the screws are positioned, if you reasonably snug them up, it is hard to keep the rotor straight on the hub. Look at the screws, they are not exactly opposite from each other. If one is too tight, it will not allow the rotor to go flush to the hub on the oposite side. If the screw is left too loose, the head can contact the wheel when torquing the lugs down and seating the rotor.

Long story short, I removed them and let the wheel and and the lug torque snug and position the rotor on the hub (like with most cars).

It has been mentioned by some that the purpose of the screws is to simply hold the rotor to the hub when the caliper is not in place, like at the factory and that they are really not needed afterwards.

Any thoughts?

Greg

 

Hi Greg,

Thanks for the ideas. Sounds like I would be better off without the screws. Is there any way to tell that rotors are aligned properly?

 

Those screws really do have a purpose. Rotors don't fit super snug to the lug nuts. So you can actually get the wheel out of balance radially? ( you know up and down). The screws will locate the rotor on the hub that is why the srew head is conical. Definately lube the threads. An impact driver is a good way loosen a stuck screw. About two good whacks and it will turn.

 

Thanks Joe,

Looks like those screws are good for something. Think I’ll keep em’. I doused them with anti-seize (as is advised for high heat areas (in this case, rust and water compound the seizing)). I tightened them alternately like I would rotate tightening on lug nuts, etc. So, I hope to have seated the rotors properly.

The impact wrench sounds like a great tool. Still the method of wedging a component, then applying force with good tools in an opposing rotation works too… see earlier post. The ¼” drive tool (from Sears) accommodates the Phillips hex bit on one end, and the ¼” drive on the other end. The impact wrench does sound like a very good investment.
<img src="graemlins/roflmao.gif" border="0" alt="[hiha]" />

 

Thomhross,

I'm going to take joe's advice and put the screws back in. I could see the rotors turn crooked when I snugged the screws last time. I'm going to try it again.

Greg

 

Hi Sharks,

Has anyone else seen rust in spots on only the inner surfaces of all 4 rotors? What could cause that?
<img src="graemlins/jumper.gif" border="0" alt="[jumper]" />