I have a 1987 f250 n/a 6.9 Diesel. The last few days my brakes have felt mushy. They work fine but different. I've had the brick hard brakes when the vacuum pump went south but never had them get mushy. They go part way down then it's like mush for a very short while then solid and no it doesn't help to pump them up. The pedal doesn't go anywhere close to the floor.

 

did you bleed the break lines that go to the wheels and there is a valve on the driver side frame rail kinda like where your feet set that needs bled to or it needs replaced

 

I have that feeling in my brakes now because I have so much play in my rear drums, the mushy feeling is the fronts sorta engaging while the rears are traveling towards engagement. Then when they finally hit you get the good pedal feel. It's terrible for panic stops.

I have found personally that if you haven't messed with them(causing the problem) then it's usually a broken rear auto-adjuster. Or both of them, like on my truck. (I am about to go fix it now, the reason it's still loose is because my parking brake cable needed to be replaced so I left it loose till I got one.)

You can pull off the wheels and see how much the star wheel on the auto-adjusters work(and if the wheel has flat teeth or a broken cable) if you aren't familiar with them. If you are familiar with them there is a little access hole you can put a screwdriver into on the axle side of the drum, down low where the star wheel is. Using the screwdriver you can tighten them up without having to pull the wheel off.

If they aren't broken and you don't set your parking brake you could still have the same problem, as the auto-adjusters only work when you set the parking brake.

-Joshua