Power brake boosters come in two main types: the common vacuum booster and the hydraulic hydro-boost. They do the same job - multiply your pedal effort - but work very differently. Here is how hydro-boost and vacuum boosters compare.
Vacuum boosters
The vacuum booster is the round unit on most cars. It uses engine vacuum to assist the pedal, so it needs strong vacuum to work; see what a booster does. A leak or weak vacuum makes the pedal hard; see hard brake pedal.
Hydro-boost
Hydro-boost uses pressure from the power-steering pump instead of vacuum. It provides stronger, more consistent assist, which is why it is common on heavy trucks and diesels that make little vacuum. It shares fluid with the steering system.
Why trucks and diesels use hydro-boost
Diesels and big-brake trucks need more assist than vacuum can provide, and diesels make little manifold vacuum, so hydro-boost is the natural fit. It delivers firm, powerful braking under heavy loads.
Servicing each
A vacuum booster needs good vacuum and a working check valve; hydro-boost needs clean power-steering fluid and proper bleeding. Both are available remanufactured; see reman vs new. Keep the friction brakes strong with quality pads and hardware.