rear brake pad gauge

[85911]

I am not familiar with the gauge referenced in the Bentley manual, page 460-8, to be used when changing the rear disc brake pads. It's called "Porsche special tool P 84." It is to be used to "set caliper piston cutout alignment."

Does anyone know what this tool does and do I need it?

I have ordered new pads from Pelican- Textar, p/n 911-352-950-06-M3.

Thanks.

[JRSIII]

I don't have my manual in front of me, but I'm assuming this is referring to the tool used to make sure the caliper pistons are properly "clocked". There should be a diagram showing the caliper, piston, and rotor, indicating direction of travel, with a little wedge shaped guide. Most people just make their own out of cardboard or sheet metal. You can xerox the page from Bentley as a template.

[85911]

Yes, the gauge sets the clocking of the piston or caliper, but I have never used one.

The diagram does is not clear on the reference points. What am I aligning the gauge to?

[Trackrash]

If the old pads are wearing evenly, you are probably OK. It is easy enough to eyeball the pistons when the pads are out.
The tool is used after a rebuild when the pistons have been R&Rd.

[85911]

The inner pad has worn much more than the outer. Is this what you mean my even wear as opposed to even, "flat" wear?

I would normally attribute this to uneven friction on the sliding surfaces between the pads and the calipers. The previous installer perhaps did not lubricate these surfaces.

Again, I am confused as to what surface I am aligning to the 20 deg. edge of the gauge.

[JRSIII]

The top of the "gage" piece is flat and will rest against the inside of the caliper. The bottom surface of the gage should then fit in the 20 degree cutout. When are you planning to do this job. I can send you a photo using some old calipers and gage I have on Thursday. Would that help?

[85911]

JRSIII: I think I understand. I have not done brakes on a Porsche yet, so I am not familiar with a "20 deg. cutout." I'm sure I will see it when I take it apart.

I'm supposed to get my pads on Thursday so will probably put them in on Friday.

Thanks for your help and offer.

[Trackrash]

It will be obvious when you get the pads out.

[85911]

Trackrash: Yes, I think so. Thanks for your reply.

[JRSIII]

If you go to the Pelican Parts online catalog and look under "brake caliper rebuilding kits" you can see a picture of the piston with the cutout. Note that on a given caliper, if one pad is wearing much more than the other, this may indicate a stuck piston and you might end up needing to rebuild or replace. You'll find out for sure if you are unable to retract the pistons to install the new pads.

[85911]

JRSIII: Yes, I see it. Perfect. Now I understand. I've never seen a cut out like this on the disc brakes I've replaced, so I was confused. It's clear now.

Again, thanks.

[85911]

Took my rear pads out this morning and saw clearly how the gauge is used. So, I went ahead and made one. JRSIII and Trackrash, thanks again.



As I stated above, the outer pads on both sides were thicker indicating a sticky piston. The left rear was the most difficult to retract, but it did move back into the caliper.

The right rear outer piston was much easier to push back than the left side outer, but only a little harder than the right side inner.

The differences between pad thicknesses are:

left: 0.101 in.



right: 0.075 in.




I may install 3 pads at a time leaving out one outer pad and gently press on the brake pedal and then push in the piston back in to see if I can free it up. I would do this several times. I would do this to both sides. Does anyone think this will work?

I have a DE next weekend and will look at the pads afterwards and if I see any differences is wear, will rebuild the calipers. All 4 boots look good.

As I stated above, I will lubricate the contact surfaces on the pads and caliper.

Is outer (rear) piston mobility a common issue on these cars (I have an 85 911)?

Thanks.

[Trackrash]

Working the pistons in and out will usually free them up. I do this when ever I bleed my brakes.

[JRSIII]

Nice looking gage! Good job. I don't have experience with your proposed method, but I suspect if you can move the pistons at all, eventually you could get them free. In my case, they were frozen solid and I ended up replacing.

[85911]

Trackrash & JRSIII: I will let you know how it turns out. My pads are supposed to arrive tomorrow evening.

Again, thanks for your help.

[JSV798]

I would replace all 4 pads. Then the pistons have all been pushed back in and are working on a fresh contact area which I would expect to give freely moving pistons. If any sticking remains you are into caliper rebuilding; not hard, I did it last year but if I do it again I would use a brake hose clamp to prevent loss of too much brake fluid.

[85911]

I installed 3 brake pads and left one out and slowly pushed down the brake pedal to extend one sticky outer piston. Then, I pushed it back in.

Did this 3 times to each rear outer piston and each easily was retracted. Hopefully, both pads will wear evenly now.

I will inspect after next weekend's DE to see if the wear is even.
I also bought and installed new wear sensors which were missing.

Thanks to all and especially JRSIII and Trackrash for your help.