BACK to High Performance 101 index page Next >>




Rear Disc Brake Conversion

By Ryan Ballou

As I've said in previous articles, making your car quicker is pointless if you have no way to safely bring it to a stop. This was the reason for upgrading to front disc brakes some time ago. Well as time goes on and more has been done to the car the question of braking has come up again. The only upgrade left is to go all out with four-wheel disc brakes by adding on a set to the rear of the car.

Once again I have Mid America Motorworks to thank for providing me with a set of their rear disc brakes. Mid America carries kits for both IRS and Swing Axle applications as well as kits with and without parking brakes. For a car that is driven on the street the kit with the parking brake is a must and therefore that is the kit type that I'll be covering. Keep in mind that when running four-wheel discs you want to be using a late model dual circuit master cylinder. Not a problem for me since my car came equipped that way.

The advantage of running four wheel discs vs. two discs and two drums is that brake fade will be eliminated in all but the harshest driving conditions, namely road racing. Wet weather performance will be increased dramatically as disc brakes shed water when pedal pressure is applied. Just try coming to a quick stop after going through a deep puddle with drums, it takes quite a bit of effort. With discs, you have full braking power almost instantly. Finally there is the little to no maintenance required, as disc brakes need no adjustment. All you need to do is monitor pad & rotor wear and replace as needed, along with regular brake fluid changes.

The kit supplied by Mid America comes with nearly everything you need for the installation. Included are a pair of rotors & calipers, a set of brake pads, caliper brackets, new rubber brake lines (soft lines), bearing housing seals, associated hardware, and a set of parking brake cables designed for this application. You will need to purchase two new brake lines (hard lines) 30" long, and if you have already gone with stainless steel braided Teflon lines for the front brakes, then you'll want to pick up a set for the rear as well. Mid America carries these Teflon lines too, they are sold per each so order two.

The first order of business is to remove brake drum. This can be tricky since the torque on the axle nut is 250 [ft*lbs] at a minimum. I use a ¾" drive breaker bar with a 4' cheater bar (my jack handle) to break these loose. The nut size is 36mm, though in some cases it may be less expensive to use a 1 7/16" socket, usually easier to find as well. Put the car into 1st gear and use wheel chocks to keep the car from rolling when doing this. With the drums removed you can take out all the old braking hardware; shoes, springs, parking brake equipment, etc.

I chose to leave the existing lines and slave cylinder connected until I was ready to install the new lines to prevent making a mess by dripping fluid everywhere. Just unbolt the bearing cap, pull the backing plate out with the cylinder still attached, and push everything back out of the way. Don't worry about bending lines, you'll be replacing them anyway.

If you haven't serviced your axle bearings in awhile, this is the time to do it while everything is taken apart. I just did mine a year ago so I'm leaving them alone for now. Clean up the bearing cap and mating surface on the diagonal arm to prevent grease from contaminating the new paper gaskets that will be used.

There are two thin washer type shims included in the kit. Deburr these completely and slide them on the axle so the sit on top of the bearing, one shim per axle. These will end up sandwiched between the bearing and the outer bearing spacer that the rotor snugs up against. With this shim installed you can set the caliper bracket in place with the mounting ear facing inward and to the rear of the car. Install the new bearing seals in the bearing caps and put the new O-rings in place (both included). Smear a little grease onto the O-ring to prevent pinching it during installation. Now set a new paper gasket in place on top of the caliper bracket and install the bearing cap back on the hub assembly. Torque each of these four bolts to 43 [ft*lbs]. With the caps in place you can reinstall the outer spacer with the tapered side in towards the bearing. Be sure to smear a little grease on it prior to installation.

Now you can install the rotors. Clean up the braking surfaces with some brake cleaner while you can still reach everything. Be sure to clean and deburr the recess that where the seal in the bearing cap will ride and smear a little grease here also. Slide the rotor over the axle and install the axle nuts hand tight for now.

Prep the calipers by installing the brake pads. These can be a little tricky, but if you look at the pictures of the install you'll see they just pop right into place. You can also install the metric adapters into the calipers at the inlet hole. Be sure to use the copper sealing washer provided so you don't have any leaks. Now you can bolt the calipers in place with the provided hardware. Use a dab of anti-seize and torque these to 50 [ft*lbs].

Almost there, we just need to install the new brake lines and we're done. If you're using standard rubber lines up front then you can go ahead and use the soft lines provided in the kit. If you're using Teflon lines up front, then use Teflon lines in the rear as well so you don't disturb the braking bias. You'll need to bend your new hard lines to fit, so I suggest going out to the hardware store and getting a small tubing bender. Go slow and test fit frequently. This is easier than you might think, but you can't really undo a bend after you make it, so measure twice, bend once.

The last thing to do before bleeding the brakes is to install the new parking brake cables. Slide the threaded end up into the tubing in the car greasing it as you go. Pass the line through the mounting boss in the caliper and secure it with the c-clip that is provided in the kit.

Adjust the cable in the car with the nuts halfway up the threads. Pull the cable tight at the caliper and you will notice that the cable is way too long to do anything. The best solution I could come up with was to cut the cable off about 2"-3" past the lever on the caliper. I used a cutoff wheel on a Dremel tool to avoid fraying the cable. Then I picked up a set of cable stops from an automotive store nearby. These look just like the stops used on the throttle cable at the carburetor. Slide the stops up against the levers on the calipers. You don't need any preload so just makes sure that both cables are adjusted to the same length in the car and you can set the stops up against the lever and secure them. I would suggest using a small propane or MAP torch to solder the ends of the cables to prevent over time.

You can now bleed the brakes. Do this just like you normally would, start at the passenger side, then drivers side. Go ahead and do the fronts while you're at it so you're starting fresh with new fluid all the way around. You should have a nice firm pedal somewhere between a quarter and half travel. Any lower than halfway and you probably still have some air in the system. It's normal to get bubbles stuck in the calipers when they are new and dry so just keep at it until you get a good pedal feel.

Once the lines are bled you can reinstall the rear wheels and lower the car. With the car back on the ground you need to put it back into gear, block the wheels, set the e-brake and torque the axle nuts, remember 250 [ft*lbs] min. Keep a very close eye on the axle nut torque for the first few weeks of driving. Between the shim that was added and the new rotor, the metal will compress slightly and that nut will become loose. I checked mine at the end of a week and it was probably down below 50 [ft*lbs]. A hard launch and I could have ripped the splines out of the hub in the rotor. Keep checking this until it stops getting loose, about 3 weeks/3 adjustments in my case.

I've already put these through the paces since they were installed. Quite a few rainy days and they performed flawlessly. No matter how much water I went through the brakes were always there. Even got to use them in a panic situation recently. Came to a complete stop from 40mph in such a short distance it was scary. Overall, this was an upgrade that was well worth the effort involved. Drive it like you stole it, but stop it like you own it.


1-I had to use one of my original rims to break the axle nuts loose since the Sport Bug rims I have now don't allow access to the axle nut while installed. Without a huge impact wrench, you need the wheel in place to help keep the car stationary.

2-With the drum removed you can see all the existing hardware that needs to come off now. Remove the shoes and springs, and unbolt the parking brake cable holder from the rear of the backing plate.

3-With the bearing cap removed the backing plate will pull off with little effort. If you're not servicing your axle bearings then be extra careful to avoid contaminating the grease while the bearing is exposed. You can install the bearing shim now. It's just stamped from a piece of metal stock so be sure to completely deburr it before installation.

4-Clean up the bearing cap, install the new axle seal and set the new O-ring in place. I'm reusing my axle seals since they were replaced when I recently serviced my axle bearings.

5-Here we have the caliper bracket in place with the bearing cap and new paper gasket installed. The steel spacer should slide in place with a little effort, make sure the taper is facing inwards.

6-You'll see the recess cut into the hub of the rotor where the axle seal will ride. You should deburr this area thoroughly and then give it a smear of bearing grease.

7-The rotor installs just like the old drums did. Slide them in place and get the axle nut as tight as you can for now.

8-This is what the pads look like when installed. You'll have to spread the caliper apart to get them in place.

9-Installing the caliper is pretty straight forward. Slide it over the rotor and hold it in place while you bolt it on

10-Bending the new hard line was a little tricky. I started at the joint to the soft line and made one bend at a time. Bend then test fit, then bend and test fit again. Keep going until you reach the rotor. Once installed you can always 'manhandle' it to clear obstructions. Be sure to check fit once the car is lowered. Mine was rubbing against an oil line in my full flow setup, but only when the car was on the ground and suspension was loaded.

11-These are the cable stops I used to hold the parking brake cable in place. Make sure they are good and tight so the cables don't slip out when you need them most.

12-Finally a little solder on the end of the cable will keep it in one piece over time.

BACK to High Performance 101 index page
Next >>

About us | Contact | Events | Forum | History | Home | Images | Interactive | Links | Tech