GMA Kelsey Hayes Rear Master Cylinder 5/8 Bore For Harley-Davidson FL FX XL

GMA Kelsey Hayes Rear Master Cylinder 5/8 Bore For Harley-Davidson FL FX XL

Precision-engineered for late-model Shovelheads and early Ironhead Sportsters, the GMA Kelsey Hayes Rear Master Cylinder is the premier choice for riders upgrading to high-performance braking systems. Featuring a 5/8″ bore, this master cylinder provides the increased hydraulic volume necessary to properly actuate most aftermarket multi-piston brake calipers, delivering superior stopping power and a more controlled feel at the pedal.

This master cylinder is a high-quality replacement for the classic Kelsey-Hayes style units used by Harley-Davidson from the late 1970s through the early 1980s. Proudly manufactured in the U.S.A., it features a sleek, compact body designed to mount directly to OEM brackets with standard 5/16″ hardware on 2.25″ centers.

Technical Specifications

• Bore Size: 5/8″
• Mounting Holes: 5/16″ at 2.25″ on centers
• Inlet Size: 1/8-27 NPT
• Dimensions: 4-1/16″ L x 1-7/16″ W x 2-7/8″ H
• OEM Cross-Reference: 42453-80A, 42454-80A
• Fluid Requirement: D.O.T. 5 Silicone Brake Fluid
• Manufacturing: Made in the U.S.A.

Key Features

• Optimized Pressure: 5/8″ bore size is specifically required for multi-piston brake setups to prevent a “spongy” feel.
• Perfect Fit: Designed to replace OEM Kelsey-Hayes style master cylinders on 1979-1984 Big Twins and Sportsters.
• Premium Finish Options: Available in Black, Polished, or Natural finishes to complement any build style.
• Reliable Performance: CNC machined for tight tolerances and long-lasting seal life.

Installation and Fitment Notes

• Fluid Warning: Use only D.O.T. 5 Silicone-based brake fluid. Do not mix with D.O.T. 3 or 4.
• Fitment Identification: This unit fits “Late 1979” models. If your 1979 model uses a Wagner-style offset mount, see our Wagner master cylinder listings.
• Mounting Hardware: Uses standard 5/16″ mounting bolts (not included).