AndrewM

OK, finished doing the rotor replacement on the 89 C4 and about 1 week later have a bad pulsating feel in the brake pedal. I then remembered that I didn't spray down the rotors with brake fluid after installing them. Could this be the reason for the pulsations? Can I just try to spray the heck out of them and hope for the best, or do I have to remove the rotors, do a light machining on them. Lastly, do I have to change the pads? Thanks...
Drew
89 C4
86 930

joey bagadonuts

Sometimes, the bumping can be caused by a residue left by the pads. Take a good look at your rotors. If you can see pad imprints, then that may be your problem.

Take a look at this white paper from Stoptech's website which may explain your problem. The site also contains an excellent FAQ section as well as other Technical information.

Arjan B.

You have to remove all the grease or paint on the rotors.

But.....did you check the wheel hub, sometimes corrosion and dirt is left on the hub. This needs to be removed before installing the new rotors. remove the dirt/rust with sand paper or some other good tool.

My be check my service topic from last couple of weeks.

chris walrod

I noticed you said brake fluid, dont use brake fluid! Use brake-clean spray. All auto parts carry it.

AndrewM

Update on this particular problem. Well, I don't think it's actually bumping that is going on, I didn't see any pad imprints or other imperfections on the rotors. I proceeded to spray each rotor (while on the car) liberally with brake cleaner. You guessed it, no difference at all, they have to be warped. I'll be taking them off next week to see how badly warped they are.. I just have no idea what could have caused this to happen on new cross-drilled rotors with about 50 miles of light use...
Drew
89 C4
86 930

dave-C2

Andrew

Please refer to Joey B's posting, it really sounds like brake pad deposition. Porsche rotors don't warp, including the cross drilled type. It's a good idea to use new pads with new rotors.

AndrewM

Dave,
Read the article and understand that the combination of not fully removing the thin oil coating in addition to not breaking in my brakes as written in that article with multiple stops from varying speeds (never had to do this before and never had a problem) has caused my problem.
Now, what can I do about it? Well, seems to me like I need to remove all the rotors and put a light machining on them. The question is can I re-use the brake pads? Just a light sanding on them and then break in the pads like stated in the article? Thanks...
Drew
89 C4
86 930

joey bagadonuts

I had the bumping after improperly breaking in a set of Pagid Orange race pads. I cleaned the rotors by using Hawk Blue race pads which I purchased from Vertex Auto. These are also high-quality race pads, so I got full use out of them but at $110/axle, it may be overkill to outfit your car with the Blues just to clean the rotors.

BTW, Drew, you should really go through the bedding in process whenever you replace rotors or pads. Here's another link from the fine folks at Stoptech which explains why this is necessary.

I've heard of people having their rotors skimmed. Maybe someone can offer a solution along these lines. Good luck.

AndrewM

Thanks Joey,
Hopefully putting a light dressing down at the machine shop will take off the deposits and allow me to "re-break" the rotors.
Drew
89 C4
86 930

Mark in Baltimore

One more suggestion: are you sure the rotor is fully seated onto the hub and that the rotor screws are fully tight?

Jeff Curtis

DO NOT TURN YOUR PORSCHE ROTORS!!

The metal is different from the domestic rotors available in the U.S. and they do NOT turn well.

How bad is the "warped" feeling??

I have a feeling that you have never had cross-drilled rotors...correct me if I'm wrong.

When you get cross-drilled rotors, you will experience a pulsating feeling, but VERY quick and light pulses, felt at the pedal under high speed breaking, you might even hear a "whirring" sound transmitted through the chassis.

This is VERY common with cross-drilled rotors, please consider if this is the case or not. MY 964 does all of the above...I'm quite used to it.

FYI - the screws that hold the rotors snug against the hub are NOT required, they are just a "neatness/perfect install" item. BUT...if you have them installed, insure they are screwed all the way in and not affecting the mating of your wheel to the hub.

I do not use them and I'm still alive and kicking after 60+ events in my 964.

AndrewM

Mark,
Everything is nice and tight rotor to hub. Jeff, the pulsating is very bad and getting worse. I have drilled rotors on the 930 and know the slight pulsing your are talking about. This is much worse, so I'm going to the garage to start the disassembly.
Drew
89 C4
86 930

Jeff Curtis

Strange, just plain strange...I POUND the HELL out of my brakes and they have yet to warp to the point that you describe.

I really suspect something else at this point, like a loose front bushing, possibly the allen bolts for one of the front shocks, wheel bearing - something other than warped rotors...it just doesn't add up.

AndrewM

Progress report.... All 4 rotors show signs of bumping, i.e. I see the imprint of the edge of the pad (looks like a bunch of lines radiating out)everywhere on all the rotors, they're going to the shop in the morning. Question is, do I need to take out the brake pads and scuff them down or just leave them be...
Drew
89 C4
86 930