Is a Hydraulic Handbrake Legal in the US?

Why?

The hydro handbrake is a motorsport tool for drift and rally, locking the rear wheels on demand through the brake hydraulics. Its defining feature is also its legal problem: it applies only while you hold the lever. State equipment codes require a parking brake that holds the vehicle when set, and inspection states test it, so a car whose only handbrake is a non-latching hydro unit fails the requirement.

The compliant install is in-line or in parallel with the factory system, keeping the original latching parking brake fully functional. That layout is common in dual-purpose street and track builds and keeps the car legal, though a visible race lever invites questions at inspection, so a tidy, professional install with intact factory equipment is the version that passes without argument.

Plumbing into the braking system is also serious work: any leak or introduced air degrades the service brakes, which is a straight equipment violation on its own. Done properly and kept alongside the stock handbrake, the hydro itself breaks no rule.