DISC

Subaru Impreza AWD Rear Disc Brake Upgrade

Project Summary:

Difficulty Level (1-5)	4 - Advanced/Intermediate
Time Required	4-8 hours
Cost	Starting at $300 for rear disc-only installation. Costs go up quickly with performance parts.
Recommendation	Recommended for serious driving, mountain driving, or if your vehicle is used often for hauling heavy loads or for towing.

This installation will upgrade the rear drum brakes on an AWD Impreza or Legacy to rear disc brakes. I performed this modification using research information obtained from the North American Subaru Impreza Owners Club (i-club) and Rallispec. The only 1993-2001 Impreza's with rear disc brakes are all 2.5RS models and certain 2.2L models in 1995 and 1996, including the 1996 Impreza Outback Sport. This documentation is based on the install on my 1997 Subaru Impreza Outback Sport, which has ABS. Your installation may differ slightly if you do not have ABS.

Note: these instructions are intended for someone with reasonable automotive repair experience and knowledge. Many details have been omitted, as they can be found in other literature such as Haynes' excellent Subaru Legacy Automotive Repair Manual (1990 through 1998). Missing details include: suspension assembly/disassembly, brake assembly/disassembly, parking brake adjustment, and brake bleeding. However, I have included pictures wherever possible to aid presentation and to provide helpful hints.

Materials Required:

1 set complete and functional rear knuckle assemblies from any 1993-2001 Impreza or Legacy with rear disc brakes, consisting of:: AWD knuckles, with hub and bearings
Backing plates
10.1" solid disc rotors
ABS sensors and cables (if applicable, and optional)
1-pot brake calipers
Brake pads (new or used)
Disc brake seal kit (optional, Subaru part number 26297AA040)
Rear disc brake hoses, Subaru part numbers 26531AA041 (RH) and 26531AA070 (LH), or aftermarket equivalents.
Parking brake cables (from Impreza RS or Legacy, but Impreza RS will be of proper length), Subaru Impreza RS part numbers 26051FA140 (RH) and 26051FA150 (LH)
Brake fluid, DOT 4
Shop rags

Tools Required:

10mm, 12mm, 14mm, 17mm, 21mm socket (ratchet) wrenches
32mm hub socket wrench
14mm deep socket wrench
12" or longer breaker bar
4" and 6" socket wrench extensions (optional)
10mm, 12mm, 14mm, 17mm open-ended wrenches
8mm, 10mm flare nut wrenches (for brake bleeding, hoses)
2-ton floor jack(s) or equivalent
2-ton jackstand(s) or equivalent
Oil/fluid catch pan or suitable container
Speed bleeders (optional)
Pliers (optional)
Small flat-blade screwdriver (optional)

Let's Start

Before jacking up the car, it is advisable to loosen the axle nuts with a 32mm socket and breaker bar. Do this with the tires on the ground and the handbrake fully applied - it will give you a much bigger mechanical advantage than when the car is up on jackstands.

Jack up the rear of the car. Use a block of wood to prevent slippage. Rest the car on two jack stands. Remove the wheels. The descriptions from this point onwards will apply only to the right side - repeat as necessary when doing the left side:

If you forgot to loosen the axle nuts and have a helper, have him/her stand on the brake pedal while you loosen/remove the nut.

Remove the brake drum. If it is stuck, insert a M8x1.25 hardened bolt into one of the holes and turn with a wrench to pull the drum out. The drum will only get stuck on high-mileage cars where the drum shoes has worn a wide groove into the drum surface, resulting in a lip in the drum that catches the drum shoes.

Drum brake internals with axle nut removed

Remove the rubber drum brake hose from the drum brake line at the strut tabs to the chassis (not shown). The rubber hose is attached to a rigid brake line out of the drum brake assembly. Be prepared to have a catch pan for the brake fluid spills (it was too late in this picture). Pinch off the brake hose or temporarily tie the brake hose so that it is up and out of the way.

Remove all the bolts securing the parking brake cable to the underside of the car. (Unfortunately, it was impossible to take pictures of the underside of the car, so you'll have to bear with line drawings.) It's probably very dirty under there, so please wear eye protection.

Go to the interior of the car, and remove the plastic center console. Remove or peel back the plastic parking brake skirt from the parking brake lever (not shown). Remove the rear seat.

Peel back the carpeting to expose the equalizer cover. You may need to remove some interior panels to get access to removing parts of the carpeting. You may also need to remove the driver's and passenger's seats to be able to peel back the carpeting.

Remove the two nuts that bolt the cable clamp to the floor.

Loosen the parking brake cable locknut and adjuster nut at the parking brake assembly. Using the extra slack from the loosened/disconnected cable, remove the parking brake cables from both sides of the equalizer. A pair of pliers will come in handy here.

Push the parking brake cables, including the rubber boot, through the holes in the tunnel floor. Fish them out towards the knuckles under the car.

Remove the strut-to-knuckle nuts and bolts. Leave one bolt in until the knuckle is ready to be removed entirely (a few steps from now).

Remove the ABS sensor from the drum assembly, if applicable. The existing ABS sensor can be reused with the disc assembly. If there are metallic particles stuck on the magnectic pickup, now is a good time to clean them off. The picture shows the sensor sitting on top of the parking brake cable, which is on top of the trailing link.

Remove the trailing link nut and bolt (not shown). This is the nut and bolt on the knuckle assembly that is lowest to the ground.

Remove the control arm-to-knuckle nut and bolt. This is the very long bolt that connects the two control arms to the knuckle. Be prepared with some lubricating spray and a lot of wrenching to pull the bolt out.

Remove the last strut bolt (from a few steps ago). Remove the knuckle, along with the parking brake cables. Gently tap the axle with a rubber mallet as you slowly pull the knuckle away from the control arms, strut, trailing link, and axle half-shaft. The axle half-shaft will slide in and out of the center differential as well, so make sure that you don't pull the axle all the way out of the center differential. You may need to use a suitable drift or punch to push the axle the rest of the way out of the wheel hub.

I used a block of wood to rest the knuckle assembly while removing it - it's heavy!

Pulling axle half-shaft out of wheel hub

Install the new (or used) brake hose. You may also use stainless steel lines such as those from Goodrich instead of the OEM ones I have chosen.

If you haven't already, preassemble the disc knuckle with the parking brake cable (not shown). Don't forget to reinstall the retaining clip that secures the parking brake cable to the knuckle.

The caliper and brake pads may be preassembled or left off for now, although it is wise to check the fit beforehand. Once fit looks good, leave the calipers off the knuckle.

Insert axle (half-shaft) into the disc hub (not shown). This is simply the reverse of the knuckle removal procedure. Apply grease where necessary. Again, a block of wood helps to hold the knuckle upright during this step.

Reinstall the control arm-rod-to-knuckle, trailing link-to-knuckle, and strut-to-knuckle nuts and bolts, in that order. You do not need to worry about alignment, as alignment is done at the other end of the control arms. The torque specs should be about 100 ft·lbs for the strut bolts, 100 ft·lbs for the control arm, and 80 ft·lbs for the trailing link nuts. Install the ABS sensor into the disc knuckle, if applicable.

Install the parking brake cable to the underside of the car, reusing the same bolts and locations as for the drum brake cable. Bring the cable into the parking brake area through the holes in the tunnel floor, and attach the cables to the equalizer. Seat the rubber boot to seal up the tunnel floor holes. Reinstall the cable clamp bolts. Note: if you have a Legacy parking brake cable, it may be too long, and you may need to loop the extra length somewhere under the car.

Attach the new brake hose banjo bolt to the caliper. Note that the OEM brake hose has a specific orientation for each side of the car, but I don't remember if the Goodrich lines are this way or not.

Test-fit the rotor disc. It's probably a good idea to turn the rotors if they have high miles on them, appear warped, or are grooved. I got mine turned for $48 at the local Subaru service department, and they did a great job. I later bought a pair of brand new rear rotors from Double Discount Auto for just $30! The industry code for non-vented rear rotors is 470.

Check the parking brake cable adjustment. You may need to turn the parking brake adjustment wheel if it's too loose or the rotor won't go in easily. Some trial and error may be required to get the parking brake pads adjusted properly.

Install the calipers and brake pads.

Reinstall the 32mm axle nut. Hand-tighten for now.

Reinstall the parking brake cable to the parking brake lever. Replace the carpeting inside the car, and reinstall the seats. Adjust the parking brake using the adjustment nut and locknut. You should be able to count 7-8 clicks until the parking brake is fully applied. Reinstall the parking brake lever skirt, followed by the center console.

If the caliper sliders are frozen or rusted, pry the caliper off the slider, clean thoroughly with brake cleaner and a cleaning pad such as scotch-brite, and re-grease with a high-temp grease. If the boots and/or seals are cracked or punctured, replace as necessary from items in the Subaru disc brake seal kit.

Replace with new brake pads if necessary. For most pre-1999 models, the industry rear brake pad code is 471. For 1999 and later models, the brake pad code is 770.

Bleed the brakes and top off with new brake fluid.

Reinstall the wheels and lower the car so that you can tighten the axle nut to no more than about 135 ft·lbs. However, if you have a helper, you can get him/her to stand on the brakes while you torque the axle nut, and not have to wait until the car is on the ground. (If you have 2.5RS wheels, the fixed center cap doesn't seem to allow access to the axle nut anyways.)

Drive carefully while the pads bed in.

I noticed that the rear rotors don't get used a lot under light braking, as normal driving barely clears away surface rust on the disc rotors. The brake proportioning valve must have a bias towards the front brakes. However, the rear discs do see a fair bit of duty when the brakes are applied heavily, and that's when this upgrade is valuable. I have also upgrade the 1-pot front calipers to 2-pot calipers, and this combination consumes rear brake pads about twice as fast as front brake pads.

I've also noticed that the parking brake is a fair bit sloppier than the old drum setup, since the rear discs utilize a separate inner drum parking brake. This is a small detail, and does not necessarily indicate a problem with the parking brakes. My guess is that the geometry of the smaller inner drum provides less mechanical advantage than the full-size drum setup.

Standard disclaimer: This information is being provided as an educational and informational service for the Subaru community. I do not make any warranties, but I will be happy to help with questions about these modifications. Please proceed at your own risk!

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