Break pedal gets hard while driving.

Bad calipers seizing up ?? Many may poo on this but before I compress the caliper I peel the rubber boot back a bit and shoot some penetrating fluid in there , done it for decades never had a problem nd calipers last the life of the vehicle . there's nothing to retract the caliper they merely relax .

Try flushing the brake system. The moisture content maybe too high so when the brakes get from just normal braking the moisture turns to stream causing the brakes to start to apply. Once they cool down the brakes release.