I second "corrosion". Once I couldn't bleed a front caliper in my 1982 Dodge Aries. The P.O.'s had done minimal maintenance and sold it because the alternator wasn't working (easy fix). Finally, I pushed as hard as possible with 2 feet on the brake pedal and a bunch of rust gunk blew out the bleeder. Hint - next time remove the bleeder. My 1982 Chevy S-10, bought new, had a rear drum lockup at 5 yrs old. The wheel cylinder had leaked brake fluid to gum up the shoe. The wheel cylinder bore was badly corroded. I envisioned the Chevy factory having an open 50 gal drum they sourced the brake fluid from, with rainwater dripping in and bad-attitude workers tossing soda cans in. I use silicone brake fluid in all my vehicles now and haven't seen any corrosion since.