Straight from Ford manual
1. Check hydraulic brake system for leaks or insufficient fluid. With transmission in NEUTRAL, stop engine and apply parking brake control.
2. Apply brake pedal several times to exhaust all vacuum in the system.
3. Apply brake pedal and hold it in the applied position. Start engine. If vacuum system is operating, brake pedal will tend to move downward under constant foot pressure. If no motion is felt, the power brake booster system is not functioning.
Also thought might give you help
1. Remove vacuum booster hose from power brake booster and check valve connection. Manifold vacuum should be available at the check valve end of the vacuum booster hose with engine at idle speed and transmission in NEUTRAL. If manifold vacuum is available to the power brake booster, connect vacuum booster hose to power brake booster and repeat Steps 2 and 3 above. If no downward movement of brake pedal is felt, replace power brake booster.
2. Operate engine a minimum of 10 seconds at fast idle. Stop engine, and let vehicle stand for 10 minutes. Apply brake pedal with approximately 89 N (20 lb
) of force. Brake pedal feel should be the same as that noted with engine operating. If brake pedal feels hard (no power assist), replace check valve and retest. If brake pedal feels spongy, bleed hydraulic system to remove air.
I would check the belt for glazing