| The car used to
demonstrate this procedure is Bob Tindels 1983 911SC, but the procedure is similar
for other Porsches.
Before starting, if
your brake master cylinder is very full, draw off some of the fluid. When you push back
the caliper pistons, if the master cylinder is completely full, it can overflow. BRAKE
FLUID DESTROYS PAINT.
Jack up the car, and put it on jack stands. If
you dont have four jack stands, you can do one side at a time, but it is a little
more time-consuming. You can put one jack stand under each rear torsion bar end cap (Figure 1), and one under each front torsion bar front end cap (Figure 2).
Remove the wheels.
Now you are ready to remove the brake pads.
There are two pins holding the pads in each caliper. The front pins have self-retainers,
and can simply be tapped out from the outside, using a pin punch or a nail (Figure 3). The retaining pins in the rear calipers have little
hairpins in the inboard end of each pin, which must be removed before the pin
can be tapped out (Figure 4). After you remove one of the retaining
pins, you can take out the brake cross-spring. Note how it fits before removal, so you can
get it back in right (Figure 5).
Remove the all of the old pads. If they are
stubborn, you may have to push back the piston first. (Figure 6)
Disconnect the brake pad sensor wire, if so equipped.
Depress the brake pedal slightly, just to the
point where the brakes would be beginning to engage, and block the pedal in this position.
This will prevent all of the brake fluid from draining out, and a lot of air getting into
the brake system, when you disconnect the caliper lines. I use a piece of wood with a
notch in one end and a small square of plywood to protect the seat at the other. With this
setup, you can move the drivers seat into position to keep pressure on the brake
pedal. (Figure 7)
Unfasten the brake line from the caliper. This
is an odd size11mm. Work carefully so you dont damage the brake line fittings.
(Figure 5 again)
Remove the caliper mounting bolts (19mm), and
remove the caliper from the car. On the rear calipers, the lower mounting bolt is
difficult to access. Working from the top, you can turn it about one flat at a time with a
box-end wrench. If you use a socket, be careful it doesnt get trappedthis is a
long bolt. (Figure 8)
This is a good time to inspect the brake lines,
shock absorbers, and rotors. (Figure 9) If the rotors have severe
ridges, or if they are worn below the minimum thickness, they will need to be resurfaced
or replaced. The same is true if they are warped or have excessive runout (a symptom of
warpage or runout is steering wheel shimmy when you apply the brakes).
Inspect the brake hoses for cracking or
leakage, and the shock absorbers for leaks. Any evidence of fluid leakage on the shock
absorbers indicates need for replacement.
If the inspection of the rotors, calipers, and
brake hoses is satisfactory, proceed with the rebuilding of the calipers. You dont
need to disassemble the two caliper halves. You can rebuild the caliper without further
disassembly (unless it was leaking at the joint between the two halves). If fully
disassembled, it is difficult to get the caliper back together without leaks.
Open the bleed screw (9mm on the front
calipers, 7mm on the rear ones), and carefully blow out any brake fluid. Use compressed
air at the brake line attachment point. You need only low pressure to do this, 15 psi or
so. Be careful where the old brake fluid goes, it will destroy paint (Figure
Clean the outside of the caliper thoroughly,
using brake cleaner spray or alcohol.
Now you are ready to begin disassembling the
caliper. Remove the dust boot retaining ring and dust boot from one of the pistons by
prying it off carefully with a small screwdriver. (Figure 11) Hold
the opposing piston in place with a C-clamp, put a piece of wood (3/4 inch or so
thickness) in where the brake pad was and blow the first piston out partway with
compressed air. Work slowly, increasing pressure until you get the first piston about
halfway out. Then change the clamp to the opposite side, remove the other dust boot
retaining ring and dust boot, and blow the second piston partway out. The idea is to get
both pistons partway out so you can get a grip on them for removal, without blowing either
of them all the way out first (if you do blow one all the way out, put it back in so you
can get air pressure in the caliper to blow the other one out). The pistons are slippery,
so use a shop rag for better grip to finish pulling them out. Dont use any tools on
the machined surface of the piston. (Figure 12)
If you can't get the pistons out using air pressure, try
remounting the caliper on the car, reconnecting the brake line, and use the brake system
itself to force the pistons out. This is an almost fool-proof method, and a good
reason for doing one caliper at a time.
SAFETY NOTE: Keep your fingers out of the
caliper at all times when you are using air pressureit builds up enormous force, and
can cause serious injury.
The next step is to remove the old rubber
piston seal from each caliper bore. I use a sharp awl to spear the seal and
pull it out of its groove. Just be careful not to scratch any machined surfaces
Clean the caliper thoroughly, using brake cleaner spray
or alcohol. Be sure to clean the piston and inside of the caliper bore thoroughly.
If the machined surfaces on the piston are heavily rusted or pitted, it must be
replaced. If there is any light surface rust, clean it off gently using a
Scotch-Brite pad or fine crocus cloth. Scrub out the piston seal groove in the
caliper bore with a small brush (a cut-off toothbrush works well).
Reassemble the caliper. Begin by lightly
coating the caliper bore, piston, and rubber piston seal with silicone assembly lube (such
as Sil-Glide, available from NAPA auto parts stores). Dont coat the dust seal. If
you cant get silicone lube, coat the parts with clean brake fluid. (Figure 13)
Insert the piston seal into the groove in the
caliper bore, and insert the piston. It should slide in a short distance without too much
effort. Dont force the piston, and be careful not to get it cocked in the bore.
Orient the piston at a 20-degree angle from the top of the brake pad slot in the caliper.
This angle is important to keep the brakes from squealing. Use a template, which you can
make from thin sheet metal or even cardboard. The template is just a triangle about 4 ½
inches long with a 20-degree hypotenuse. (Figure 14 and in the
After you are sure the piston is going in
straight, and is at the correct angle, push it the rest of the way in with a piece of
wood. If you get the angle wrong, blow the piston back out, and start again. Check the
angle of the piston at least twice before going further. It is frustrating to get a
caliper mounted on the car, and then see that the angle of one of the pistons is wrong.
Here is the factory manual diagram of how to check the piston angle.
After making sure the piston angle is correct,
install the rubber dust boot and retaining ring. The front caliper pistons should be
pushed all the way in before installing the dust boots. On the rear calipers, leave the
pistons sticking out about ¼ inch to make installing the dust boots easier (the boot fits
like a turtleneck sweater on the rear pistons). The dust boots should be clean and dry on
the outside after you finish installation. Repeat the installation process for the second
Remount the calipers on the car. Check the
piston angle one more time. (Figure 15) Torque the caliper
mounting bolts to 70 ft/lbs.
Now you are ready to install the brake pads. If
you are reusing old pads, make sure the braking surfaces are clean and free from any oil
Do not use the anti-squeal compound pad from
your Friendly Local Auto Parts Store on the back of the brake pads. This stuff turns to
glue, and when you need to replace the brake pads again, it will destroy the caliper dust
boot when you pull the pads out (dont ask how I know this). Most pads now have an
anti-squeal substance already applied at the factoryit looks like a thicker coat of
paint. If you are going to use anything on the backs of the pads I recommend a LIGHT coat
of Lubro-Moly Hi-Tack Lube Spray. Be careful not to get this stuff on the braking surface
of the pads, or on the rotors.
Insert the new pads the same way the old ones
came out. Reconnect the sensor wires. Insert one of the retaining pins, taking care that
the hole for the hairpin in the retaining pins of the rear calipers is
accessible. Be sure to tap each pin in until it is fully seated. You should be able to see
the end of the pin in its hole on the outside of the caliper. Insert the
hairpin into the brake pad retaining pin.
Insert the brake cross spring, and then the
second retaining pin and hairpin. Make sure that both ends of the brake cross
spring engage the retaining pins. The cross spring helps retract the pads, so they
dont drag on the rotors when you let off the brakes (Figure 16).
Repeat steps 2-15 for the other three calipers.
You are getting near the end now. Top off the
master cylinder. Use only fresh fluid from a sealed container. We recommend a top-quality
fluid such as Ate.
Bleed the brakes using your favorite method. I
like to start the bleeding process at the wheel furthest from the master cylinder, and
finish with the wheel nearest. The important thing after major brake work is to do the
entire bleeding process at least twice to get all of the air out of the system. Tapping on
each caliper with a mallet can help release trapped air bubbles. When you finish, you
should have a hard brake pedal that engages the brakes at 30-50 percent of its travel. You
may need to bleed the brakes again after a few days of driving.
Here is a picture of a pressure bleeder I made
using the directions at http://www.garageboy.com/bmw/bleeder.html.
I bought the parts at REI and a hardware store. My friendly local tire store gave me the
valve stems. (Figure 17) It is a neat, clean way to bleed brakes
or to pressure-flush all the old fluid out periodically.
If you use a vacuum bleeding system, such as
the Mity-Vac, put some heavy grease around the base of each bleeder screw, where it screws
into the caliper. Otherwise, it will leak around the bleeder screw, and make it look like
there is still air in the system.
Reinstall the wheels. The correct torque for
factory alloy wheels and alloy lug nuts is 95 ft/lbs.
Give yourself a big pat on the backyou
just saved several hundred dollars in labor (try calling a Porsche dealer and get an
estimate on rebuilding your brake system. The quote I got for this work was $2,433,
assuming your rotors are good).
CAUTION: New brake pads
require a break-in period. Until they are broken in, braking efficiency is reduced. Apply
the brakes gently during break-in to increase the life of pads and rotors.
It is a good practice to flush and bleed the
entire brake system at least every two years. Brake fluid absorbs moisture from the
atmosphere, and over time can cause expensive parts to rust beyond repair.
Call our Parts Department at 1-888-280-7799 and
we will fix you up with the best brake parts for your usestreet or track (and some
fresh brake fluid if you need it).