Any 770-1570 can have this brake problem if contaminated oil if left to circulate through the hyd system. Looking at the brake valve you will notice the inner spool in the LH port is not sticking out like the one in the RH port. It is stuck in because of metal getting passed the bypass valves in the filters of a 1070. The filters had not been serviced for years. Often when this occurs in the brake valve the powershift pilot spools are sticking also like this tractor. When the brake spools are stuck in, the brake with the problem will engage whenever backpressue is created in the system like turning the steering wheel. I found the source of some of the contamination. Rod.