Bob Prosser

Hey all. A month ago I replaced the rear rotors on my ’87 911. A simple, straightforward project. The pads were fairly new, so I didn’t change them out – just the rotors.

Here’s my problem: During the last few feet as I roll-up to a stop light, I can detect a gentle surge effect. It’s not through the pedal – it’s a seat-of-the-pants thing. It’s as if the brakes are slightly warped, but I haven’t done any hard driving yet. I don’t feel the surge (pedal or seat) in hard or moderate use, say 60-30 or 30-0 MPH – only as I roll-up to a stop.

I’d suspect the front rotors, which are still original, but before I did the job there was no pulse whatsoever.

Anyone else ever felt this after replacing rotors? Think I should have replaced the pads?

Thanks in advance or any input.

Chris Martin

I have had similar issues with other cars. If the rotors are not oem quality they sometimes can warp pretty quickly, especially if they get wet.

Also, like you mentioned you should have replaced the pads too, in my opinion. Street pads are so cheap you might want to replace the pads and see if this helps your problem. Check the simple things like tire pressure and any tire or wheel wear just to be sure as well.

Good luck

Howard

I know this may sound a little obscure, but is it possible a: there may be grease or oil on the rotors from the install? I always clean the rotors with acetone to remove any grease, oil or brake fluid that may have ended up on the rotor surface.
or B: the rotors are installed over the studs then retained with a flat head screw, is it possible there is some debris between the new rotor and hub causing runout?

Just a thought, I know it's not likely, but worth investigating. You could check runout with a dial indicator if one is available.
Good luck !

Bob Prosser

Now Howard, that first suggestion is a damn fine one! There could have been some manufacturing oil still on the rotor. I was in a hurry. I inspected them and wiped them down but did NOT wash them in solvent. It's worth doing now.

I don't think idea "b" is likely -- but it's certainly worth noting. Runout isn't that hard to detect.

Bob

ron mcatee

When changing rotors, I always put new pads in. The old rotors set the pad angle and it may be much different than the new ones. Unless you ride them hard to wear them in (which isn't likely) they will continue to wobble or pulsate. My suggestion is to remove the disks and clean with Acetone or other cleaner as stated previously and put beand new pads in it. Take care not to "burn in" the pads by getting up to 90 MPH and trying to stop on a dime. For the first few miles go easy with them to wear them in and seat them properly .

Some may say this is BS, but it worked for me on two occasions.

Hope you get it fixed soon.

Spider911
88 Carrera, White/Red

Jeff Curtis

Bob, when changing rotors...and keeping the current pads intact, I "scrub" the pads on my cement garage floor. The "grit" level of my floor is fairly fine, so it doesn't score the pads up, it actually does a nice job of putting a fresh "scrub" on them.

I wouldn't attribute your "pulsing" effect to the fact that you used your old pads. I wouldn't attribute it to the protectant oil that's on the rotors either. If you wiped them down whatsoever, that's good. USUALLY, the oil burns off after the rotors warm up during your first use anyway.

You might want to do a few good hard brakes, of course, in an area where it's safe. I do it back in an industrial park, near my development.

Mind you, I change pads and rotors quite often, as I attend MANY track events, sometimes two a month.

MY suggestion for the "pulsing" sensation you're feeling is a unique one...so bear with me:

What I think may be happening is that ALL of your rotors were warped, in one form or another. You replaced the rears, so now they're nice and smoooooth. Your fronts are still warped...so the fact that the rear braking is smoooooth, it magnifies the fact that the fronts are warped.

Did that make any sense??

I would replace the front rotors and see what happens.

Bob Prosser

Thanks Jeff. Hate to replace the front rotors just yet because I replaced the front wheel bearings last weekend. Had those rotors in my hand but didn't replace them because they didn't measure under spec. Oh well.

I will try some hard braking and get the brakes nice and warm -- without getting a ticket or going shiny side down.

Bob

Roland Kunz

Hello

I´m with Cris & Spider911. Reusing used pads can make trouble and ruin the new disc.
However the 3,2 brake system isn´t as critical on that like the Turbo or the brembo caliper systems.

None mentioned that your problem, might be that your brake caliper pistion is moved out from position. Better check if they have a 20° angle onramp.

Grüsse