brake pedal issues

https://acornmetric.com/s/9WtpnwacGjvP7ylh-e0RE

My brake pedal feels off—sometimes it's spongy and sinks to the floor, other times it's stiff and unresponsive. The garage said it might be air in the lines or a failing master cylinder, but I'm worried it's the brake booster. Last week, I had to pump it frantically to stop at a red light, and my mechanic warned me not to drive until he replaces the fluid and bleeds the system. It's unsettling knowing your brakes might give out mid-turn.