Hello all! This is my very first post to the site so might I just say cheers!
I have a 1986 f250 4x4 6.9 with an ats turbo and 4-speed manual. When I bought the truck it had this problem. I replaced the proportioning valve and vacuum pump and flushed the system but the problem is only getting worse. I don't have any front brakes and the brake pedal is very squishy. Also, parking brake doesn't work very well. Any advice is greatly appreciated. Thanks!

 

For the parking brake cable, there is no way to fix it really, just have to replace if theyre froze up. Have you checked the calipers / pads in the front? Usually soft front brakes is from the rubber brake lines going soft and expanding as you push on the pedal.
For the soft pedal, make sure everything is bled out good, and the rear brakes are adjusted properly. To adjust rear brakes, use a flat head screwdriver, or equiv. and turn the adjuster inside the backing plate, this is accessed through a little oval in the bottom of the back of the backing plate. Pop out the rubber gromet and you should see a cog looking thing about 1" in diameter with teeth. I dont remember which way the adjust so trial and error will tell you. Have the rearend in the air and truck in neutral. Spin the tire as you adjust, clicking is normal while adjusting that is the auto adjuster. Adjust brakes until you are no longer able to turn the tire by hand, then back off until there is a only a slight drag on the brakes. Repeat for the other side.

If that dont fix it, its master cylinder time. Make 110% you bench bleed the hell outta that thing too.

 

I've already had the breaks adjusted but I think it might be the front hoses like you said. Ill look into it... Thanks for the help!