I'm having issues with the brakes on my 97 F150. It has rear abs. The master cylinder ,wheel cylinder and calipers have all been replaced. At times the rear brakes grab hard and other times the pedal is hard but the brakes aren't grabbing. Also if I am applying pressure lightly coming up to a stop, the pedal will suddenly just go to the floor, but if I hit the brakes hard they work just as they should. Any ideas would be helpful.

 

Your master cylinder is bad... slow apply pedal to the floor means the seals aren't fully sealing.

When you jab it hard you overcome the slow leak in the seal and energize the seals, thereby sealing up the leak.

I know you said you replaced it, but it sounds like the one you got either has bad seals or contamination.

If it is contamination (dirt under the seal) you could try bleeding it to see if that works. But if its new I would just return it for a different one and tell them the MC is bad.

As far as the pedal being hard sometimes... is the engine on when you are doing this? (or are you sitting in your garage with the engine off?)
if the engine is on then it sounds like you have a vacuum leak... check your vacuum hose to the booster for leaks.

 

I've actually have tried 2 new master cylinders, as for the pedal being hard this with the engine running. I have ghecked the vacuum hose and it is good. I also forgot to mention that the abs light will come on after about 10 minutes of driving, but if I leave the truck running the light doesn't come on( I've let it set for 30 minutes). Does the abs valve need to be bled? if so what is the best way to do this?

 

There is air trapped in your ABS pump. You may have to have the dealer auto bleed the pump with the NGS computer or be creative....Find a empty road with grass on the side and drive the two right side tires on the grass and jam the brakes on so the ABS pump comes on....then go back and bleed...u may have to do that a few times but it gets the air out of the pump....hope that helps.