Brakes & Rotors

Brakes are noisy!

The ugly fact of engineering is brake caliper systems are noisy, all of them, period! The challenge of the engineers of these systems is to minimize caliper/pad/rotor noise within the human hearing audio spectrum.  This might explain why your dog seems to launch into a sing-along

aria when you touch the brakes on your car for no apparent reason. Controlling brake noise is very hard to do as brakes being a dynamic slipping friction clutch application have a lot of variability in what is going on when you press on the brakes. Knowing this, at best one can only try to minimize brake noise and even then with highly variable success. This is why brake noise is the number one complaint mechanics receive related to brake work, "hey that's not a problem, that's a feature".  My experience is that if one uses the relatively soft OE dealer or aftermarket pads and replaces the spiders it will not eliminate but will likely minimize the occurrence of brake squeal. Also on a 993 as the rotors approach their minimum wear specification brakes can be a bit noisier.

Don't look for love too hard, try this first!

Recently a friend of mine was experiencing brake squeal with his 993 at low-speed stops. On his car brake pad and rotor wear seemed to not be the issue. He resolved the problem by making a series of aggressive stops at a relatively high speed of 40~60mph. His problem may have had to do with the pads not being fully bedded to the rotors, the added heat adding to the curing process of the brake pad friction material, or any number of other root or contributing factors. In any event, having heard this if your brakes are squealing at low-speed stops and the pads and rotors seem to be in good shape I would try heating up the system in this way before investigating the issue any further.

Brake warning light. The reason the light comes on is when a sensor clipped to one of the brake pads is ground down to the point that it becomes an open circuit.

Option to eliminate the brake wear sensors:

On my car, I soldered the sensors' wires together, insulated them, and tie-wrapped them out of the way against the struts. If one visually inspects their brake pads every 30K~40K, something that happens every year or two in many states due to mandated state safety inspections, there is no need for the expensive sensors.

Replacing Brake Pads

A Wear Sensor

Pads need to be replaced when the friction material looks to be thinner than the backing plate. It can be difficult to observe the friction material directly with the wheel on. I use 2 popsicle sticks held together as a feeler gauge to check the friction material thickness without taking the wheel off.  Pads worn to the minimum or rotors running near their maximum wear limits tend to contribute to greater instances of brake squeal on the 993. 

Replacing the brake pads can be done with the calipers in place or removed.  Replacing the pads is relatively easy however Porsche specifies that if you remove the front calipers you must use new bolts to remount them. I do not know definitively why this is needed but Porsche wouldn’t specify this unless they saw a failure in a test so I just do it. Taking the pads out without removing the calipers is easy however one needs to use a 1” metal paint spatula to slice the squeal silencers, sometimes called spiders or noise clips off the pads' backing plate so the pads can be removed.

Some replace the spiders when installing new pads some don’t. If you don't use spiders when mounting the brake pads you may experience greater brake noise. You will also hear an occasional clicking sound at times as the pads are not held firmly in place.

A brake pad Spider

The price of new silencer/spiders is high at the dealer and seeing that one will need 16 of them for our cars the total cost can add up to $200. A good inexpensive source for the common-sized ones is rockauto.com where they are priced between $3 and $6 each for the various sizes needed or about $80 for a complete set for all four wheel-ends.
For my 1996 narrow-body Carrera-2 here are the ones I ordered recently from RockAuto.com:

4 each of the below part numbers for all 4 wheel-ends:

96535109600 (351 096 00) - 44MM - Front

96435109601 (351 096 01) - 36MM - Front

96535209601 (352 096 01) - 34MM - Rear

99635208600 (352 086 00) - 36MM - Rear

Other models will need different combinations of sizes as listed in the parts catalog.

Reusing Spiders;

The spiders from Rock Auto seem to have thin sheet metal mounting discs vs the older ones, I seem to recall, came with rubber discs. For this reason, I see no reason why the newer metal spiders can not be reused. This is done by cleanly slicing them off the backing plate initially as part of removing the old pads with a 1" metal spatula. Then if needed the bending of the metal tines at their center out just a little so they snap in cleanly when reused.

When reapplying them hose them off with a little spray brake cleaner and use some 3M™ Adhesive Transfer Tape 468MP on the spiders' disk face as an adhesive to reapply the spiders. This tape is rated for use up to 400 Degrees F. 

An alternative is to use the readily available brake anti-squeal compound/adhesive for example CRC

05016 Disc Brake Quiet. If one uses this liquid compound type adhesive apply it to the spider's mounting disk face sparingly smoothing it as a very thin layer over the spider's disk and not to the very edge of the disk, and let it dry a bit before clipping the spider into the caliper cup. Doing this will avoid it oozing out from under the disk and gluing the backing plate to the caliper piston cup seals. By using care in the application of the adhesive compound the seal is not damaged the next time the brake pads are removed.

Selecting pads for a quiet ride:

Pads from the dealership are staggeringly expensive so if shopping for aftermarket pads that will run quietly look for very soft organic pads if you can find them or alternatively street pads that list low noise as an operating feature such as Centric's Posi Quiet pads or similar from other suppliers. In most cases, they are either organic or semi-metallic pads intended for street use and should be reasonably quiet. The operative word being "should". Pads that are marketed as performance pads or pads for track use are usually made of a harder compound, designed to run at higher temperatures, and generally are more prone to squealing noise. I have used pads from the dealer, Textar, and Centric/Stop Tech. It seems that how quiet a pad runs is a little hit or miss.

Little things to make changing the pads a lot easier:

• If the brake reservoir is nearly full or full I draw it down a little with a turkey baster or a syringe before working on the brakes. After installing the pads on each wheel end I pump the brake peddle a little pushing the caliper pistons against the new pads and rotor. In this way it avoids the cumulative effect of all the wheel ends having all their piston cups fully pushed in causing a good volume of brake fluid to back up into the reservoir potentially overflowing it and making a mess.
• I compress the caliper piston cups using a "C" clamp while the old pads are still in place. Placing it along the top edge of the caliper casting and the pad backing plate. If you have a concern about scratching the paint on the caliper face one can use a little patch of rubber to protect it from the "C" clamps jaw.
• A small but key item when removing the caliper hold-in bowtie spring is to detach the little metal

  

  Detaching the Sensor clip

  
  clip that holds the wear sensor wire from the center of the bowtie spring before squeezing the bowtie to release it. The clip seems to interfere with compressing the bowtie just enough to make it difficult to pop the bowtie free from the caliper housing.
• The two spiders' discs must be sliced free from the backside of each pad to release the pad. I found using a one-inch metal painter's spatula and sliding it behind the pad from the center of the pad at an angle toward each spider seems to release them readily.
• When installing new pads with spiders without removing the caliper I find pealing off the spider's adhesive backing plate covers first and snapping the spiders into the piston cups before guiding the new pads into position seems to be the way to go.

  

  Squeezing the Bowtie

  
  Using the spatula seems to facilitate this. Once the brakes are cycled for the first time the adhesive will be pressed firmly against the backing plates. 
• If one of the spiders seems to not snap in smartly and wants to fall out just bent out the little tines on its metal center clip a little and it will hold fast when snapped in. 
• It may be necessary to slightly champer the inner curved edge of the pad friction blocks to provide clearance to get the pads to slip into the caliper easily.
• I also chamfer the leading and tail edges of the pad's friction blocks very slightly as it seems to minimize squealing early on with a new set of pads.

Cleaning up the look of the wheel nuts and calipers at your next brake pad change: 

While you have the wheels off consider applying a quick shot of gloss black paint

to the worn anodizing on the nuts. Only the top of the nuts' caps really needs to be painted completely as it is really the only part one can readily see when the nuts are installed. As I worked my way around the car replacing the brake pads I give a shot of paint to the nuts for that wheel. By the time I am ready to remount the wheel the paint is dry enough to handle the wheel nuts with a little care. 

Cleaning up the look of the calipers:

Also if your calipers look dull and the clear coat has chipped off one can

restore them as they work their way around the car using the method on this page.

Evaluating rotors for replacement:

The official way is to measure the thickness of the rotor to make sure it is greater than the OE minimum specification of 30mm in the front and 22mm in the rear. The shade tree approach is if there is a good size ridge on the outer edge of the rotor where the pad does not touch the rotor face they should be considered for replacement.

Ridge formed at the rotor's edge.

Keep in mind there is only a 2mm overall difference in rotor thickness between a new rotor and one that has reached its wear limit. This translates into an approximate 1mm ridge having formed around the circumference of each face of the rotor.

OE supplier to Porsche drilled rotors can be purchased online at Rock Auto at a surprisingly affordable price. If you took the calipers off to replace the pads one need only remove two rotor hold-down screws to replace the rotors. The screws are best removed with a manual impact screwdriver, an inexpensive tool purchased at Harbor Freight.

Impact Driver

I always paint the contact surfaces of the rotor to the hub and the hold-down screw with anti-seize when mounting rotors as it will make the rotors easier to remove in the future. The same goes for the rotor contact points on the wheel hub.

Here is a detailed procedure for replacing rotors.

Styles of Aftermarket Rotors:

A question that is regularly asked is are the drilled (original design), slotted or plain-faced rotors best. Although the engineers designed the braking system thinking about rotor weight, pad heat gassing, and air path for cooling, I feel, the brakes on a 993 have significant excess capacity beyond that needed for street use and speeds so the style of the rotor probably doesn't matter much for street use. Now for cars that are raced, I would go with the factory configuration as it has been extensively tested for performance and reliability. Also, the configuration of the fins internal to the rotor is very specific to optimize cooling so for high-performance driving I would stick to rotors from the suppliers who originally supplied rotors to the factory at assembly or rotors supplied specifically for racing applications on the 993.

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