GMA Engineering Kelsey Hayes Rear Master Cylinder 5/8 Bore for 1979-1984 Harley-Davidson FL FX XL

GMA Engineering Kelsey Hayes Rear Master Cylinder 5/8 Bore for 1979-1984 Harley-Davidson FL FX XL

When a high-performance caliper upgrade demands more than a stock master cylinder can deliver, the GMA Kelsey Hayes puts the right hydraulic authority behind every stop. Designed to work in concert with multi-piston aftermarket calipers, it gives riders the firm, controlled pedal feel that stock bore units simply cannot provide — backed by the precision of American CNC manufacturing.

Key Features

• 5/8″ bore delivers the hydraulic volume required to properly actuate multi-piston aftermarket calipers without a spongy pedal feel
• CNC machined from aircraft-grade heat-treated aluminum for tight tolerances and long-lasting seal life
• Mounts directly to OEM brackets using standard 5/16″ hardware on 2.25″ centers — no modifications required
• Available in Natural, Polished, and Black finishes to suit any build style
• Proudly manufactured in the U.S.A.

Technical Specifications

• Bore Size: 5/8″
• Inlet Size: 1/8-27 NPT
• Mounting Hole Diameter: 5/16″
• Mounting Hole Spacing: 2.25″ on center
• Length: 4-1/16″
• Width: 1-7/16″
• Height: 2-7/8″
• Fluid Requirement: D.O.T. 5 Silicone Brake Fluid only
• OEM Cross-Reference: 42453-80A, 42454-80A
• Manufacturing: Made in the U.S.A.

Fitment Notes

• All 1979 fitment applies to late production only; early 1979 models using a Wagner-style offset mount are not compatible — see GMA Wagner master cylinder listings
• 5/8″ bore is specifically required for multi-piston brake setups; use a 3/4″ bore unit for stock single-piston calipers
• Use D.O.T. 5 Silicone-based brake fluid only — do not mix with D.O.T. 3 or D.O.T. 4
• Mounting hardware not included
• Review the Fitment tab for the complete year and model breakdown