How to Test a Power Brake Booster | YourMechanic Advice
If your brakes start to feel spongy, the root cause may be the power brake booster. Test your power brake booster to see if it needs replacement.
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Turn the engine off, then repeatedly press the brake pedal slowly. When you pump it the first time the pedal should be very 'low'— meaning not much pressure resistance. As you pump the pedal, the pressure should become firmer, which will indicate that the brake booster is not leaking.