Drum to disc brake conversion: what the job actually needs

Numbers matter on a brake job, so this guide does not invent them. Bore sizes, bearing preload, torque figures and bleeding order vary by car and by kit. Get those from the factory service manual for your car and from the instruction sheet that comes with the parts. Where something is a genuine judgment call, this guide says so.

Two cautions before you start. Linings and drum dust on a car of this age may contain asbestos, so never blow the dust out with compressed air and never dry brush it. Wet the parts down with brake parts cleaner, wipe them into a rag, and bag the old shoes and the rag rather than sweeping anything up. And put the car on jack stands on level ground with the back wheels chocked before any part of you goes under it, because a jack on its own is not enough.

Decide if the conversion is the right fix

Drums are not dangerous when they are set up right. A front drum system with fresh linings, round drums, free wheel cylinders and working adjusters stops a street car about as short as the tires allow. On one hard stop from normal speed, the gap between good drums and discs is smaller than most people expect.

What drums do badly is repeat. Heat builds inside the drum and has nowhere to go, so the second and third hard stop get longer and the pedal gets softer. Drums also pull when one side grabs differently, which happens with a weeping wheel cylinder, a contaminated lining or a seized adjuster. They hold water after a puddle and take a moment to dry out.

So convert for consistency, not for a number on a stopwatch. The other honest reason is parts. Pads, rotors and calipers are easy to find. Correct drums and matched shoes for some cars are getting harder to source.

What a kit contains, and what it leaves out

Most front kits give you rotors, calipers, pads, caliper brackets, hardware and dust shields. Better kits add spindles, bearings, seals, dust caps and flexible hoses. Read the parts list before you buy, not after the box arrives.

What kits usually leave out is the half of the system that decides whether the car stops straight: the master cylinder, the proportioning or combination valve, the residual pressure valves, the hard lines, and wheels that clear the calipers. Some kits keep your original spindles, others require theirs. Some change the bolt pattern, which means new wheels as well.

Lay everything out and compare it to the car before you take anything apart. Ordering the missing pieces up front is what keeps this a weekend job.

Master cylinder and residual valves

A drum and drum master cylinder is the wrong part for a disc front. It normally holds a residual pressure of roughly ten psi in both circuits to keep the wheel cylinder cups seated. Leave that pressure on a caliper and the pads stay lightly loaded, so the brakes drag, run hot and glaze.

Use a master specified for front discs and rear drums, in the bore the kit maker calls for. Bore size sets pedal effort and travel, so follow their instruction sheet rather than guessing. If the master sits below the calipers, as with a floor mounted unit, a two psi residual valve in the front circuit stops fluid draining back and firms up the first pedal. If the new master has no ten psi valve for the rear drums, add one in the rear line.

Bench bleed the master before it goes on the car. It is far easier on the bench than under the hood. Once it is mounted, check the pushrod length. A pushrod that is too long holds the piston past the compensating port, so fluid cannot return to the reservoir; the brakes then apply themselves as the fluid heats and expands, and the car can come back with locked wheels and a cooked hub. You want a small amount of free play at the pedal before the pushrod starts to move the piston, set to the figure in your service manual or the kit sheet. If the wheels get harder to turn after a few stops and the pedal creeps higher, suspect the pushrod before anything else.

Proportioning valve and rear bias

Front discs and rear drums do not want the same pressure. Discs need more, drums need less, and too much at the back locks the rear tires first, which is how a car ends up sideways in the wet.

Many conversions use a combination valve, which packages three jobs: proportioning down the rear pressure, a metering function that holds the front off slightly so the rear shoes come in together with the front pads, and a pressure differential switch that lights the dash warning lamp if one circuit fails. The other common approach is an adjustable proportioning valve in the rear line, which lets you tune bias on the car.

Set bias on a clean, empty surface at low speed, in small steps, with nobody around you.

Booster or manual

You do not need a booster. Plenty of these cars stop well with manual discs, as long as the master bore and the pedal ratio suit each other. Manual setups use a taller pedal ratio than power setups, so a power pedal with a manual master gives a hard, high effort pedal that feels like something is wrong.

A vacuum booster wants steady manifold vacuum. A stock or mild cam usually supplies it. A cam with a lot of overlap may not, and the symptom is a firm pedal that gets worse after two or three stops in a row. Put a vacuum gauge on the engine before you buy, then compare the reading to the booster maker's stated minimum. If vacuum is short, an electric vacuum pump or a hydroboost setup solves it.

Driver and car matter too. A heavy car and a light driver argues for power assist.

Wheel clearance and backspacing

This is the step that catches people out. Calipers sit outboard of where the drum backing plate sat, so wheels that fit yesterday may not fit today. Smaller diameter steel wheels are the usual casualty, and so are wheels with a lot of positive offset.

Mock it up before you finish or paint anything. Bolt the rotor and caliper on, hang the wheel, and turn the hub through a full rotation at ride height with the steering swept lock to lock. Look for contact at the caliper body, the bleeder screw, the bracket and the wheel weights. Check the tie rod end and the lower control arm too.

Some kits change track width slightly, which moves the tire closer to the fender lip. Check that at full lock and full droop, not just sitting still.

Bearings, seals and hubs

Kits come two ways. A hubbed rotor carries the bearings and the wheel studs, so you fit the whole assembly. A separate hub and rotor lets you reuse your hub, but only if the bearing sizes and stud pattern match what the kit expects.

Either way, fit new bearings, new grease seals, new cotter pins and new dust caps. Pack the bearings by hand or with a packer until grease comes through the far side of the rollers, using a high temperature wheel bearing grease. Confirm the races are fully seated before you drive a bearing home, and keep everything clean; grit in a fresh bearing undoes the whole job.

Set preload by the procedure in the factory service manual or the kit instructions for your car. Do not guess at a torque figure, and do not run the spindle nut tight.

Lines, hoses and fittings

Old rubber flex hoses fail from the inside and can act like a one way valve, holding a caliper applied until it cooks. Replace them as a matter of course. Check length and routing at full lock and full suspension travel, because a hose that only just reaches at ride height will tear.

Hard lines often need reworking, since a new master or valve puts the ports somewhere else. Bend with a tubing bender, cut square, and flare properly. Most American cars of this era use an SAE double flare, though some use a bubble flare, so confirm what your fittings take before you cut. Never put a compression fitting in a brake line.

Use flare nut wrenches on the fittings so you do not round them, and replace any line that is pitted or scaly rather than fighting it.

Bleeding, adjusting and the first drive

With the master bench bled and installed, bleed the system in the order your service manual gives. On most cars that means starting at the corner furthest from the master, but some systems with a combination valve call for a different order, so check rather than assume.

The pressure differential switch can shift off center while you bleed and block flow to one circuit. If a corner will not bleed and the warning lamp is lit, look up the re centering procedure for your valve; some need a tool to hold the piston.

Use fresh fluid from a sealed container and do not reuse what came out. Pick one fluid type and stay with it, and do not mix silicone fluid with glycol fluid in the same system.

Before the car moves, hold firm pressure on the pedal for thirty seconds, or have someone hold it, and go over every flare, hose end, bleeder and reservoir seam with a light and a clean rag. Any weep is a failed joint and will not seal itself. The pedal has to stay put under that steady pressure; if it sinks slowly, you have a leak or a master that is bypassing internally, and the car does not get driven until you find it. Then adjust the rear drums before you judge the pedal, because slack rear shoes feel exactly like air in the lines. Bed the pads using the sequence the pad maker specifies. Do the first stops in an empty lot at low speed, checking that the car tracks straight and the pedal holds, before it goes anywhere near traffic.

Common questions

Are my drum brakes unsafe as they are?

Not if they are in good order. Fresh linings, round drums, free wheel cylinders, working adjusters and a sound master cylinder will stop a street car about as short as the tires allow. The weakness is repeat stops and pulling, not a single stop from normal speed. If your drums are healthy and the car gets light use, rebuilding them is a reasonable choice.

Do I need to convert the rear to discs as well?

No. Front discs with rear drums is the standard setup on most cars of this era and it works well. The front does most of the braking. Rear discs mainly add parking brake complication and cost. Spend the money on the front conversion, good tires and a correct proportioning valve first.

Can I run manual disc brakes without a booster?

Yes, as long as the master cylinder bore and the pedal ratio suit each other. Manual setups use a taller pedal ratio than power setups, so bolting a manual master to a power pedal gives a hard, high effort pedal. Follow the kit maker's recommendation, and consider power assist if the car is heavy or the usual driver is small.

Will my original wheels clear the new calipers?

Sometimes, and you cannot tell by looking. Calipers sit outboard of where the backing plate was, so smaller diameter steel wheels and wheels with a lot of positive offset often foul. Mock up the rotor, caliper and wheel before you finish or paint anything, and turn the hub through a full rotation with the steering swept lock to lock.