Aftermarket Disc Brake Conversions
A classic truck means an older chassis. Many of our favorite trucks came with four-wheel drum brakes and a single master cylinder. Whether the goal is higher performance and a more nimble chassis or simply wanting safer, modern brakes, many owners turn to disc brake conversions. For cruising and mild highway driving, front disc and modern rear drum brakes work well.

If your project is an off-frame restoration or buildup, the use of four-wheel disc brakes can provide a technology improvement. There are contemporary OEM and aftermarket rear disc brakes that incorporate the emergency brake.

Aftermarket kits are available from several of the CLASSIC TRUCKS advertisers. Some kits are cost-conscious, using OEM-style rotors, a master cylinder, and calipers with mounting brackets and assembly hardware. For the competition-minded builder, there are upscale brake kits with high-performance calipers, rotors, and hydraulics.

Ventilated and cross-drilled rotors have been popular in high-performance circles. Under extreme braking, some brake pad materials tend to "outgas." This phenomenon creates a gaseous layer at the surface of the pads. Gas is not a friction surface, so disc brake performance suffers. Outgas and cooling were the original motives for cross-drilled rotors. Cross drilling does have a downside: Under extreme use, drilled holes can lead to stress cracks.

Another improvement is the slotted rotor, which sheds dust, water, and gas. As a racing measure, slots have the added benefit of minimizing glaze on pads. The slots scuff pad faces keep hot lining from glazing over or "vitrifying." Slotted rotors go through a set of pads quickly, not an issue at the racetrack but problematic for the street-driven vehicle.

When constructing a disc brake system, pay attention to the hydraulic system. The master cylinder and calipers must match up. Rotor size and caliper design will determine stopping force. A "kit" from a reputable source, or a complete system from a similar donor vehicle, can take the guesswork out of your parts list.

One overlooked factor when selecting the master cylinder is residual pressure. Typical drum brakes have wheel cylinder cups with lips that flare outward. If system pressure were to leave the cylinder, sealing lips could collapse and leak fluid. A traditional solution has been the use of a check valve within the master cylinder that holds residual pressure in the lines after the pedal releases.

This pressure is not excessive. Since drum brake shoes have stiff return springs, there is counterforce to the residual pressure. Fluid stays trapped in the wheel cylinders at a pressure below that of the shoe return springs. The shoes stay clear of the drums until the next brake application. An added benefit is that the wheel cylinder is fully charged after the brakes retract. Pedal travel will immediately move the wheel cylinder's shoe links at the next application of the brakes.

Disc brakes, by contrast, require no residual pressure. The piston seals continuously, even with the brakes released. Disc pads ride next to the rotor, so brake pedal apply pressure will immediately create braking force. The disc brake system does not require residual hydraulic pressure. In fact, residual hydraulic pressure could cause brake pad drag against the rotors.

Some racing disc brake systems maintain a slight amount of residual pressure, just enough to keep the hydraulic system primed. An OEM method of charging the disc brake system is a "quick take-up" master cylinder, which dumps high-volume, lower-pressure fluid into the system immediately upon brake application. This gets the disc pads to the rotor faces quicker.