Here's a simple test for the brakes to determine whether to look into your brake booster or master cylinder:
With the motor off, pump the brake pedal until it is firm, Once it's firm, hold the pressure on the brake pedal.
Continue holding pressure on the pedal and start the engine. If the pedal does not sink down then the problem is your brake booster. If the pedal does sink down then I would look into your master cylinder
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