I've noticed this happening slightly more frequently. Originally it was so rare that I wasn't even sure if I was imaging it or not. But over the past 6 months I've noticed that about 5-10% of the time when I brake the car will pull to the left. It's often quite subtle, like even without holding the steering wheel it's a slight drift. But occasionally, even with holding the steering wheel I'll be able to feel a slight pull in the steering wheel - to the point where if I let go it'll relatively quickly drift into the other lane.

Could it be the right brake pad not applying enough pressure? Or the left brake pad applying too much? Last time I rotated tired I did notice that the left brake pads had about 40-50% life left, and the right had about 60-70%. And there are some parallel grooves on the left rotor whereas all other rotors are smooth. Has been about 5 years since I changed the brake fluid.

Hard to quantify how much it pulls to y'all, but I think if I wasn't super attentive, and just some average driver, I would have never noticed.

Curious if anyone has any thoughts on first steps to mitigate it.

I've had this happen, years ago, and new brake pads fixed it. One set had "glazed" over for lack of a better word (some type of contamination, or probably just crappy pads) and it changed the braking force between the fronts.

Before changing pads and that scarred rotor, I would bleed the calipers to make sure you don't have air in the calipers (most likely the passenger side as you noted), and check for leaks around the caliper, look for fluid behind the pad or on the inboard pad. If you see fluid coming from the caliper piston(s), that's your issue and you need a new caliper. If you replace pads, I'd do both sets in the front.

Check tire pressure and condition first, look for uneven wear or pressure being off. And then You'll need to look at brakes and suspension.

What kind of road surfaces have you noticed it happening on? I had a massive pull to one direction on a hard stop on one of the many grooved highways in the Houston area last year that made me think there was something very wrong with my truck. After testing hard stops 50-10 mph on a bunch of different surfaces it turned out it only happened on those heavily grooved surfaces so I just chalked it up as another reason to hate driving in Houston.

Roger350 said:

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What kind of road surfaces have you noticed it happening on? I had a massive pull to one direction on a hard stop on one of the many grooved highways in the Houston area last year that made me think there was something very wrong with my truck. After testing hard stops 50-10 mph on a bunch of different surfaces it turned out it only happened on those heavily grooved surfaces so I just chalked it up as another reason to hate driving in Houston.

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Good question. I'm not entirely sure since it's hard to reproduce. It feels like it occurs more on rougher roads, and perhaps it's worse on rougher/deep grooved surfaces. But sometimes when I'm trying to reproduce I'll tap the brakes and notice it, and then I'll let go and press the brake a few more times and these times it'll be normal.

I'm going to rotate my tires this week or next and see if it changes anything.

If it's an older car, the right rubber brake hose to the caliper may have broken down internally and is clogging. This happened to me many years ago. Rebuilt calipers, pistons and pads didn't fix it. Replacing the stupid rubber hose did.