first check the wheels and make sure they are adjusted up till the wheel just starts to drag on both sides (do not over adjust, the drag must be slight)
Also while adjusting the wheels check the outside of the backingplates and see if there is any fluid comming out at the base just inside under the truck, if you see fluid on the backing plates under the truck (usually gets on the tires) then you have a leaking wheel cylinder and it will need replacement or rebuild.

then go to the master cylinder and check the pedal freeplay, it should not be more than 1/2 to 3/4 of an inch freeplay before it contacts the actuating rod.
make sure it has plenty of fluid in the resivoir and there is no fluid comming from the boot. If it is leaking then replace the master cylinder.
If there is no adjustment needed at the rod then try bleeding out the system and see if there is air in it. If there is no air but you still cant get a good pedal there might still be an internal problem in the master cylinder, replace it.

good luck