concor

My car has been rejected because of rust on the inside of the rear brake discs. About half of the surface has rust like the brake pad never touches it.

I ordered new discs and pads. Is there anything I should pay special attention to while installing these due to the earlier rust problem? Thank you.

I have a '84 928S with the standard brakes in the rear, starting with the 944 part number.

Landseer

Buy and install caliper seal kits.
Make sure the sliding surfaces for the calipers are fresh.
On dis/reassembly, make sure bleeders are clear / not clogged.

Consider replacing brake lines too.

Easy job, takes an afternoon. I've rebuilt 6 sets this way, front and back, and my S brakes are awesome. By doing this process, you don't have to worry about pistons hanging, incomplete bleed nor brake line restriction.

heinrich

Seems like your caliper may be sticking. Fixing this, may be as easy as carefully manipulating it until it moves freely. Do not over-extend the hydraulic parts, just get them to move. Carefully use lubricant after cleaning.

GlenL

It's odd that the rust is on the inside. The piston is on te inside and moves but the siding parts freeze so the outside is rusty.

Maybe there's no pad on the inside?

Something is odd. Removed the caliper and see what's up.

concor

Thanks for the tip Landseer and Heinrich! I didn't thought I needed to clean the sliding parts of the two caliper parts. So I cleaned them and put some copper grease on the caliper sliders as you can see in the photo. The orange stuff.



Below you can see the disc. Only the center lane is without rust and worn and thin. The inside and outside lanes are thicker but rusty. The brake pad is worn on the outsides, the center lane of the pad has a bump that fits the rotor. Very strange.

This forum rules by the way, I was wondering if I should disassemble the caliper and some older post from 2003 mentioned it was pretty hard to get it back together, so luckily I didn't. I just carefully slided it up and down, with a rubber hammer.

I installed new discs and pads back together and it seems to work nicely now.

Mrmerlin

I am glad you got the car inspected those rotors look oorible . \
From the picture posted rebuilding the calipers might be a smart thing for you to do.
I would also venture a guess that you need new E brake shoes as well

concor

Good guess, I was putting the wheel bolts on at 125 Nm with the car of the ground on the handbrake. And on one side the wheel turned before I reached 125.

heinrich

The full story is that they failed it not for rust, but because the disc was disintegrating.

123quattro

I changed out the rear brake bias valve on my 84 for a 33 bar unit. The factory one, 10 bar?, barely supplied enough brake pressure to even keep rust off the disk in general use.

31-TL-ZS

Hey Conor, so this was the reason for a lousy lap time in Assen?