I have a 1994 Ford f250 with 280,000 miles. Second engine, third tranmission. It has developed a problem when braking with a trailer in tow. The trailer weighs approximately 8,500lbs when loaded. I pull three trilers all with electric brakes and the truck does this with all of them. The truck wants to pull hard to the left when the brakes are applied quickly. I can barely notice if it pulls when I have no trailer in tow. I have replaced both calipers and hoses, rear pads, a leaking wheel cylinder, and replaced the steel brake lines on the back axle. I have not totally ruled out the proportioning valve, however, I feel as long as I am getting fluid and pressure on both axles, the valve should be good.
I am 90% certain the problem is not in the braking system. I was curious if anyone else had experienced this problem. My thoughts are turning to maybe a loose transaxle or something else in the front end. Any thoughts or insightful wisdom would be greatly appreciated.