You must have a leak somewhere and be losing fluid. Check all wheel cylinders. Brake pipes and master cylinder for leak's.
POWER BRAKE BOOSTER
Start engine. Turn ignition off. Depress brake pedal several times. Depress pedal firmly and hold pressure for 15 seconds. If pedal sinks, master cylinder, brake line or wheel cylinder is faulty.
With pedal depressed, start engine. If pedal sinks slightly, vacuum unit is working properly. If pedal height does not vary, booster or check valve is faulty.
With engine running, depress brake pedal. Turn ignition off. If pedal height does not change while depressed for 30 seconds, vacuum booster is okay. If pedal rises, vacuum booster is faulty.
Stop engine, and depress brake pedal several times using normal pressure. Pedal height should be low when first depressed. On consecutive applications, pedal height should gradually rise. If pedal height does not vary, check power brake booster check valve.
Check Valve Test
Disconnect power brake unit vacuum hose at booster. Start engine, and allow it to idle. Ensure vacuum is available at booster end of hose. If vacuum is not available, vacuum source or check valve is faulty. Repair vacuum source or replace check valve, and retest.