Something as simple as low air pressure in the left tire can make it pull to the left when braking. Alignment will affect it also. Your rubber brake hoses can also deteriate to the point that they affect the flow of fluid to the caliper. And pepper is right, do it in pairs. If it's never been done, it wouldn't hurt to install new rubber hoses and flush the system with new brake fluid anyway. It's usually worth it in the long run, especially in a situation like yours where you already have issues with the whoa pedal.