Need Brake Issue Advice
 

97 F150 4x4 5.4L
:noob:
Yesterday on my way home from work, my brakes all of a sudden went soft, to the point where I actually said 'woah!' out loud. I drove on, and noticed I had quite a bit of pedal movement before I got any real grab. At one point, I had to hit the brakes fairly hard and all of a sudden the brakes got hard again and jerked the truck with a loud thud. After that, they slowly got soft again. This has happened one more time since, referring to the hard grab. The brakes stay real soft otherwise. I still have to go to work so today I babied it and there was softness but no grab but I bet if I hit them hard it would have. I have been searching forums for info but have not seen anything mentioned like what I am seeing. Any ideas? I don't see any leaks and my reservoir is 'safe'. There also has not been any work done on the truck lately.

You may have rusty or stuck guide pins in the bracket not letting caliper move freely.The brake pads may be rusted in the brackets not moving freely. Got to check it. Remove the front calipers,check both guide pins move in/out freely and with a screw driver try to see if pads come out and in of the brackets easily. Same with the back calipers. If not, that's your problem. Master cylinder going back can cause a low pedal but I don't think that's your problem at all! Check everything else first that I said above.

BAD not back!

probably a bad MC... if a very fast brake apply builds pressure then you are likely apply so fast that it overwhelms the leak in the MC seal.

Even though I got rid of the truck I am updating this for the record and future searchers..the metal brake line rusted through on the driver side rear wheel. Brakes tend to do all kinds of silly things when all the fluid blows out. Ended up running a new line from the master cylinder back.