Few concerns : help

[lowrider284]

So i was on my way home from work today and went to brake when i was getting off of the highway when all of a sudden the brake pedal became hard to push down and the truck wouldnt brake well. I just had my rear calipers replaced at ford about a month ago and when i got home i saw that none of the calipers were smoking, i also checked my brake fluid and it was fine. Any insight would be great. Also my coolant level is about 1/3 of an inch of the minimum line. No white residue on the tank either.