Hydraulic brake ssist units are powered by hydraulic pressure from the power steering pump. These units do not require a vacuum source and can generate between 1200 to 2000 psi to the calipers. This system has several advantages, such as not requiring a external vacuum pump for high performance motors, a small bore master cylinder is not required, and firewall & valve cover and/or frame mounted clearance is increased. Frame mounted hydraulic brake assist units are roughly the same width as a standard dual master cylinder, thus allowing additinal room for routing your exhaust. Note: assist units are rebuilt.