The brake pedal on my Silverado has started feeling soft and spongy, and it goes further down than it used to before the brakes really bite. It's not failed completely but it's clearly not right, and I'm nervous about driving it much further. I'm near ICAD so I've been looking into Expert Chevrolet service and brake work in ICAD, but I want to understand what's likely causing it before I take it in. Is this usually air in the system, a failing master cylinder, or a fluid leak? And is it safe to drive short distances while I sort out a booking, or should I be parking it until it's looked at? I'd rather be cautious than find out the hard way that I pushed it too far.