Dont confuse hydraulic brake fluid with common hydraulic oil. One is mineral based the other"Brake Fluid" definitely is not. Just a few drops of mineral oil will ruin all the seals in master cly and wheel cylinders. The hydraulic brake booster is pump driven the same type of system as a power steering pump and can use the same "power steering fluid". Just realize that the hydraulic brake BOOSTER system is separate from the hydrauic BRAKE system.
I"m not telling you to do it but many people inter change between automatic trans fluid and power steering fluid; problem being there is at leat 3 different types of automatic trans fluid. Using hdraulic oil is hardly worth the savings especially when you are talking BRAKES.
Frank