There isn't a dedicated check valve *on* the vacuum line itself to the power brake booster in a Ford Freestar (or most vehicles). The check valve function is usually integrated *within* the brake booster itself. The booster contains a one-way valve that prevents engine vacuum from being lost back into the intake manifold when the brakes are released.
If you're having brake problems related to vacuum, the issue is likely *within* the booster, a leak in the vacuum lines, or a problem with the vacuum source (e.g., intake manifold). A separate, externally visible check valve is not typical in this system.
