Remote brake boosters are brake boosters that are not mounted to the firewall by the driver's side behind the master cylinder. Due to either space constraints, or the need for additional braking power, remote brake boosters are mounted elsewhere on the vehicle and then plumbed back to the master cylinder. The two general types in widespread use are remote vacuum boosters and remote hydraulic boosters.
Remote Vacuum Boosters
Remote vacuum boosters have remained in use for decades in passenger and commercial vehicles -- particularly in trucks not equipped with air brakes. These traditional brake booster systems have no mechanical connection between the brake pedal and the booster. To operate, they require a vacuum source. These boosters can supplement braking power on vehicles with weak brakes or on vehicles (such as commercial trucks) that tow heavy loads.
Complications from Remote Vacuum Boosters
The downside to these systems lies in the fact that bleeding (replacing brake fluid) is a tricky process since it does not allow you to use typical vacuum bleeder. Bleeding such systems entails filling the master cylinder and then pumping the brakes, which forces the fluid through the lines. You must perform this task with the engine off and no vacuum present in the system. Bleeding such systems requires a second person: one person working the brake pedal and the other operating the bleed screw at the remote booster.
Remote Hydraulic Boosters
These boosters are typically used in custom cars that have engine swaps that make it impossible to mount a traditional brake booster on the firewall. Such vehicles have a hydraulic remote brake booster mounted outside the firewall (for instance, on the bumper well) and then plumbing running back to the master cylinder. These boosters require no vacuum, as they run on pressure from the power steering pump.
Complications from Remote Hydraulic Boosters
Since these remote boosters rely on the power steering system for power, they require you to run additional lines to the power steering pump, in turn charging the pump with both steering and brake duties. In addition, these systems cause a parasitic drain on horsepower when used.