In hot-rod slang, "juice brakes" are what type of brakes?

In hot-rod slang, “juice brakes” are hydraulic brakes.

The nickname refers to the brake fluid, or “juice,” that carries pressure through the system. Pressing the pedal moves a piston in the master cylinder, forcing fluid through sealed lines to wheel cylinders or calipers, which apply the brake shoes or pads.

This was a major upgrade for many postwar hot rods. Earlier cars commonly used mechanical brake linkages, whose rods, cables, and levers could require adjustment and deliver less consistent braking. Hydraulic systems distributed pedal force more evenly and made four-wheel braking more practical.

A common mix-up is calling juice brakes “disc brakes.” Hydraulic describes how force is transmitted; disc describes the rotating brake hardware. Hydraulic systems can operate either drum brakes or disc brakes. In the hot-rod context, “juice” specifically contrasts hydraulic operation with older mechanical brakes.