Brake problem

[Mosin603]

Wierd thing happened to my brakes starting today. Have a 99 4wd 4wabs 3.0l. If I ease the truck to a complete stop with slight brake pressure vehicle is fine. However on a hard stop the pedal goes from a nice hard pedal to a soft pedal....almost like a line blows. While this is occurring the vehicle pulls to the driver side a little. After about 3 seconds the brake is hard again, but still all the way to floor. Once pedal springs back braking is returned until the next hard stop. Kinda feels abs related. Don't see any fluids from master, reservoir full. Any suggestions?