Brake pedal feels very hard to press
Braking requires much more leg effort than usual
Engine idle changes when the brake pedal is pressed
Hissing sound near the brake pedal or firewall
Brake pedal travel feels abnormal
Brakes respond slowly or weakly
Stopping distance increases
Check engine light or brake warning light is on
Engine stalls or runs rough when brakes are applied
Booster vacuum hose is cracked, loose, or disconnected
One-way check valve is damaged or leaking
Fluid or moisture is present inside the booster
Brake pedal does not return smoothly
Pedal feels inconsistent between presses
Booster test fails when the engine is off and then started