you could also have a bad right side brake line hose. I had this problem on one of my cars. I would check the caliper or wheel cylinder on that right side.
check the left side lower control arm bushing, it is likely shot. mine was on my 66 coupe. also make sure that your brakes are properly adjusted. that said until you swap over to a dual master cylinder you will always get a slight pull to the left as that side will always apply slightly sooner than the other side.
I think it is probably the lower control arm bushing that was mentioned.
It could also be a bad tire. I have had that happen before. You could try rotating the tires and see if it goes away. If it does that tire has an issue.