Some vehicles don't use a vacuum assist for the brakes. This might be due to space considerations or because the engine being used does not produce a lot of vacuum, such as with a diesel engine. These systems often use hydraulic pressure from the power steering system to assist with brake application. If you have difficult power steering or have a hard time applying the brakes, you might have an issue with your hydroboost unit. Don't forget to inspect the lines as well as the power steering pump for failures. Here's a video on the replacement of a hydroboost unit in a GM van.