1995 Ford F150 2 wheel drive 5.8L engine 120000 mi

Soft brake pedal. My brake pedal suddenly seems to mush down to almost the floor. I have bled the system at least 3 times and no change. If I have to brake suddenly I get full braking power and pedal stays up, but if I brake slowly or ease off the pressure on the brake then the pedal once again mushes down again. Now just today, I get a vibration and I get a sound like a dog giving a moaning howl as I push the pedal down. Desparate for any help !!