A rubber brake hose can fail internally and restrict the flow of fluid to that brake caliper or wheel cylinder also,so if the left front one was not letting equal pressure be applied,then the right front wheel will have more braking power and make it pull to that side--a caliper piston seizing up or the caliper failing to slide properly on its mounting bolts can do this too..
If the front end alignment is off,it could only show up while braking too.
I had a puzzling pulling to one side issue on a '79 C10 I had--I replaced the hoses,pads,and it still pulled to one side while braking..
I then decided to replace the calipers,and I found out there are two different ones depending on the GVW ,one has a bigger piston,but still took the same pads as the smaller version,and will bolt up the same..the truck had one of each on it,someone must have swapped on a used one ,possibly off a C20 ,and being mis-matched,it caused the truck to pull when I applied the brakes..