Odd brake lock-up issue's

[rn240sx]

Ok i have a 92 nissan pickup and im having issues with the front brakes...

When its cool or cold outside i have NO problems.... at all..!! When its warmer / hot out that when the problem arises...

I start by leaving work at 4pm and get about 5 miles out and notice that my brake pedal will start to get harder and harder to the point where i barely tap it 1/4 inch and the brakes will engage that fast... within a mile or 2 more the front brakes will engage to the point where i can leave the motor in Drive, but it wont go forward cause the front brakes are engaged.... I have to stomp on the pedal to get the truck moving and literally give it about 50-75% throttle to get it to descent speeds. By the time i get home, i can smell brakes all over the place, pull up into my slanted driveway and put her in N and she wont roll back down at all cause the front brakes are engaged.... meanwhile the brake pedal is stiff and wont push down anymore than 1/4 inch....So i leave it in N and put bricks about 1 foot behind the rear tires and just let it sit there... it usually takes about 3-4 hours b4 the brakes will let go and the truck will slide down the driveway till the tires hit the bricks and the truck stops....

This is a daily routine and ONLY does it when its hot out, NEVER in the mornings nor when its cold out....

I know its the front brakes cause the 1st time it happened, i got home, jacked up the front end and i could NOT spin the tires with my hands... the brakes were obviously locked up for whatever reason i dont know....

i tried to bleed all 4 to the best i could and didnt notice any air in the system, but i did notice that the brake fluid wasent exactly clean / pure like new brake fluid, but again, i dont know if that brake fluid should look new or if thats normal for brake fuid brake down over time and look like that...

any help.. any theories...??