The Domino Effect from Dumping Drums
The advantages of disc brakes are well known to serious four-wheelers. Although discs have lessarea than comparable-application drums, they work better when hot and wet. Plus, discs are easier to service.
Many older 4x4s can be updated to disc brakes using later-model OE components or aftermarket kits. Slapping on rotors and calipers is the easy part of the job; getting all four corners to function properly can be challenging. Half-assed disc conversions can actually stop worse than the drums they replaced, and some "kits" don't include all the components necessary to optimize stopping. The solutions aren't as easy as upgrading to higher-tech pads to increase stopping power....
To put it simply, disc braking takes foot pressure and translates it into enough clamping force to stop a multithousand-pound vehicle. The focus here, to use an engine analogy, is dialing in "naturally aspirated" disc-brake conversions for pre-ABS 4x4s. Vacuum-assists and hydro-boosts "supercharge" brake systems by increasing pressure, but they rely on a running engine to generate vacuum or hydraulic pressure. Just as adding a forced-induction unit to a tired engine puts the proverbial cart before the horse(power), a dialed-in manual braking system should be even better if swinging pedals and booster-mounting space are available. In other words, strive for brakes that'll keep a stalled 4x4 from rolling down a hill.
Doing some math might save time and unnecessary parts later. Start by finding your pedal ratio (normally about 5:1 for manual systems and 3:1 for power-assist brakes; see diagram):
pedal ratio = pivot point to foot pad distance / pivot point to master-cylinder rod distance
When upgrading brakes, a hard pedal can indicate that the ratio is too low for the new system; too much pedal travel means that the ratio is too high.
Next, get your master cylinder's specifications (from a parts-store counterman's catalog or an aftermarket specialist if necessary). Pre-'66 vehicles should always be updated to a dual-reservoir master cylinder so that one end will theoretically stop in case of leakage or failure with the other end.
OE 4x4 master cylinders' strokes are usually between 1 and 2 inches of piston travel, and their bores are typically between 7/8 inch and 1 1/8 inches. Rear calipers require more pressure and volume than drums' wheel cylinders, which is why drum/drum or even disc/drum masters often won't work with four-disc systems. Drum setups also have residual pressure valves in their master cylinders. These valves maintain about 10 psi of preload on the wheel cylinders so that the shoes won't retract completely when the pedal is up. If attempting to retain a drum-system master cylinder, remove the residual valve(s) from the outlet port(s). Otherwise, disc-drag will result from the residual pressure. Further, manual master cylinders have a deeper piston hole than vacuum-assisted units, and their bores normally have a maximum 1-inch diameter to limit the force required to move the brake pedal.
If the pedal ratio and pivot point are mismatched to the master cylinder's stroke, the system might not achieve full pressure and the piston might not retract all the way. A partially depressed piston can create brake drag even when the pedal is all the way up, and a pedal that hits the floorboard before full piston travel occurs won't achieve full braking pressure.
Front disc brakes typically require 1,000-1,200 psi of hydraulic pressure to fully actuate (around 800 psi is rock-bottom for disc movement compared to 400 psi for drums). A system's pressure capacity can be calculated:
master cylinder pressure = pedal force / piston face area (sq. in.)
So, a 100-pound person "standing" on a brake pedal with a 5:1 ratio moving a 1-inch-diameter master-cylinder piston ("pi" symbol R2 = 0.7854) yields about 637 psi of braking power (500 ? 0.7854). Swapping in a bigger-bored master cylinder moves more fluid but generates lower pressure. Conversely, a smaller bore creates more pressure but moves less fluid. It also requires more pedal travel, so a smaller-bore master cylinder might not be the pressure solution if the pedal is mounted close to the floorboard.
So, changing a master cylinder's stroke and/or bore alters the amount of fluid the system displaces. Volume remains constant (glycol-based brake fluid can't be compressed, and hydraulic braking systems are "closed"), and the pressure rises when the pedal is pressed, causing the calipers' pistons to move. Larger-bore master cylinders require a shorter stroke to displace the same amount of fluid as ones with smaller bores. Here's the fluid formula:
fluid displacement = master cylinder's stroke length (piston cylinder) x piston surface area (sq. in.)
If each front caliper's pistons displace 0.075 cubic inch and each rear displaces 0.05 cubic inch, our total piston area is 0.25 cubic inch. If we double this for a safety factor to allow for seal-swelling, deflection, and potential leakage, we need 0.5 cubic inch of total cylinder displacement to move all four caliper pistons. If our master cylinder's stroke is 1 1/8 inches (1.125), we arrive at 0.5 ? 1.125 = 0.444 square inch of required master-cylinder piston area, or approximately a 7/8-inch bore (0.444-inch radius x 2 = 0.8888-inch diameter, or about 7/8 inch).