front brake help...

[westong]

Working on a 1972 K10......since i got the truck the front brakes have been poor.realy poor actually...when on gravel or a hard stop on pavement the rear wheels lock up and it feels like the front are weak. I replaced both front calipers and still no luck, when bleeding i have good fluid coming out with no air in all 4 corners and can get the front to lock up when tryig to turn by hand. I have bleed all 4 five times with no diffrence....old calipers acted the same way...any ideas.....thanks