Fitting Rear Disc Brakes to a Front-Wheel Drive

This article was first published in 2004.

If you’ve got a base model front-wheel drive, chances are that it came with drum rear brakes. Drum brakes – while quite serviceable – don’t have the braking capability of discs and even in a car that’s not being driven hard, they have another downer. They look terrible, especially if you’ve got large diameter alloy wheels that make them even more visible.

But help is at hand. Not only can you probably source disc brakes but you can also fit them at home. In this story we look at fitting discs to the rear of a 1999 Toyota Prius, using ZE122 Corolla hardware. But much the same procedure will be used in any Toyota swap, and these pics will also give you a very good guide as to what to look out for in other makes and models.

Before you can do the swap (easily, anyway) you need to find a car that has the same rear suspension but came out with factory discs. It doesn’t have to be an identical suspension, but the hubs and axle carriers need to be a direct swap. Easiest is to find the same make and model that was available either with rear drums (your car) or with rear discs (the ones you want).

However, as we did here, you may be able to find a completely different model that has bits that are interchangeable with your car.

Removing the Old Brakes

The first step is to place the rear of the car on axle stands and then remove the rear wheels. Don’t support the car with just a jack – use proper stands and chock the front wheels.

In most cases the brake drum will then pull straight off – although in some cars a few screws may need to be undone first. Here a rubber mallet was used to loosen the drum....

...which could then be removed.

This car has ABS, something which is very important to take into account in this swap because the ABS sensor is built into the back of the bearing. We’ll come back to this in more detail in a minute, but at this stage the ABS wiring plug (arrowed) needed to be disconnected from the sensor.

Next the handbrake cable needed to be disconnected. To access it, one of the shoes needed to be first removed. This required undoing a spring clip...

...and then accessing the end of the cable which could then be undone with pliers.

After that, the bolt that retains the handbrake cable in place could be undone...

...and the cable slipped out through the back of the assembly.

The brake hose could be undone next. These nuts are very easy to round off so a specialised brake hose spanner (that looks like a ring spanner with a small section cut out) may need to be used.

With the connections between the brake assembly and the body of the car undone, the whole hub assembly could be unbolted from the suspension. Lubricating spray was squirted over the rear of the hub...

...and then the four retaining bolts were undone.

The assembly should then just slide off but in this case the assistance of a rubber mallet was needed.

The complete assembly – hub, bearings, ABS sensor and brake backing plate could then be lifted away.

The backing plate and the hub assembly were separated using a vice, some silicone spray and the trusty rubber mallet.

Removing the New Brakes from the Donor Car

The complete rear suspension assembly of the donor car was purchased. Normally, you’d just buy the brakes but in this case it was expected (wrongly, as it turns out) that other parts were going to be used to perform other modifications. The suspension bits you can see here cost AUD$400.

When disassembling the donor brakes, take particular care to see how they are different from the brakes you are replacing. And only ever do one side of the car at a time – that way, if you need to back-pedal, you can reassemble things using the other side of the car as a guide.

The brake caliper was removed by undoing two bolts.

With the caliper off, the disc could be pulled off the hub.

To remove the handbrake cables, the inner parking brake (as you can see here, they’re drums) needed to be partially disassembled. This photo was taken not only for you to see what it looked like, but also so I could find my way back to this position when the inner parking brake needed to be reassembled.

In this car the handbrake cable is the same whether drum or disc brakes are fitted. You should check that this is the case, and if it isn’t, get the handbrake cable that matches the new brakes.

With the cable disconnected, the assembly could be removed from the axle and then...

...the hub separated from the backing plate.

Mixing and Matching

The original hub/axle carrier is on the left and the new one is on the right. As can be seen, the original has an ABS sensor and the new one does not.

The ABS sensor bolts to the end of the axle – this view is from the rear with the cap removed. However, the donor disc brakes use a different axle system that doesn’t have a nut on the inner end of the axle. So the ABS sensor couldn’t be unbolted from this assembly and transplanted to the new assembly. Instead, the whole assembly (complete with ABS sensor) was used with the new disc brakes.

The unit was cleaned-up with a wire brush before...

...being installed with the new disc brakes backing plate and caliper mount.

However, when the bolts that holds the assembly to the suspension were being done up it was realised that they were a little short. What was going on? Here it was useful having both complete suspensions at hand. Close inspection showed that the Prius has a strengthened rear hub carrier – there’s an extra piece of steel welded in. The disc assembly is also thicker and the combined outcome is that slightly longer bolts are needed.

To pass through this greater thickness, new high tensile bolts were bought. Note that bolts used in suspensions must be high tensile, available from specialist engineering and bolt supply shops.

The original hub and the new disc brake backing plate were then bolted into place.

Fitting of the handbrake cable was the reverse of its removal from this disc brake assembly. It was threaded through the backing plate....

....then attached to the handbrake drum assembly...

...before being held in place with the original bolt.

The disc could then be placed over the handbrake assembly...

...and the caliper fitted.

The brake hose from the new brake assembly was then connected to the brakeline.

Not shown here (because there weren’t enough people around to take photos and bleed the brakes at the same time!) is the brake bleeding. This is a vital step.

The handbrake shoes will probably need to be adjusted before the wheels are placed back on. Make sure that you follow a workshop manual if you’re at all unsure of how to adjust the handbrake. Usually, a specified shoe/drum clearance is obtained at the brake end of things and then the number of notches that the handbrake lever moves is adjusted at the handbrake.

Done!

The result is a firmer pedal, increased braking capability and a much better through-wheel appearance!

Important Points 1. Brake Bias If the slave cylinders in the new brakes have different characteristics to the originals, you may find that the brake bias is no longer correct, ie the rear brakes lock too easily. If this is the case, further changes may need to be made to components in the brake system (eg using the proportioning valve from the donor car) or the master cylinder may need to be swapped for the one used on the all-disc model. One way to see if this is likely to be the case before you start work is to compare the parts numbers of the master cylinder and proportioning valve (if one is fitted) of the all-disc and disc/drum versions of your car. If a different master cylinder or brake bias valve is used in the all-disc car, you should budget to buy these as well. In the case of the swap shown here, no other braking system changes needed to be made. Brake bias valves can also be modified to provide a bias different from standard – see Brake Bias. 2. Pedal Travel It’s possible that pedal travel will increase excessively after you’ve done the swap. In this case you’ll need the master cylinder from the disc/disc car. 3. Brake Drag If the rear brakes drag slightly after the swap has been done (easy to know as they’ll get hot even when no braking has occurred), there are three points to check. Firstly, make sure that the master cylinder pushrod is backed-off far enough that the fluid can easily flow back into the cylinder. Drum brakes have strong springs that pull the shoes away from the drums, but disc brakes do not – the result is that the master cylinder pushrod may need to be adjusted so that there’s sufficient reverse flow capability. This adjustment is normally made at the brake pedal and is easy to do. The brake light switch will probably also need to be adjusted. Secondly, in some cars a ‘residual pressure valve’ is used on the drum brakes. This maintains a small pressure in the rear brake lines all the time, allowing the slave cylinder seals to better do their work. However, on disc brakes this pressure can cause the brakes to drag. To see if either of these is occurring, place a clear plastic hose on a rear caliper bleed valve and open the valve after the brake pedal has been applied and released. If fluid comes out of the caliper when the valve is opened, there is some pressure being retained in the rear brakes. If there is, adjust the master cylinder pushrod first then if the pressure remains, check for the presence of a residual pressure valve. This valve is most commonly located at the rear brake exit port of the master cylinder. Finally, brake calipers that have been sitting in the rain at the wreckers for a few years may have become a bit sticky. In this case, a caliper rebuild kit may need to be put through them.

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