I have a similar issue with my 48 truck. Didn't really think too much about it because i'm not to familiar with old hard riding 3/4tons with big wheels and skinny tires.. When i did motor mounts and front harmonic balancer sleeve w/seal i cleaned 70 years of crap covering the brake line going to r-front. I now remember it is dented/kinked. Can this line be not pressuring the right brake cylinder correctly and be making the left brake overworking? I didn't think much about my issue till i read ur conundrum.
u can see the line here, u can't see the dent as its down and behind the c-support.
