I like PB blaster penetrating catalyst spray. I'd try some vice grip pliers on them.
Here's a situation I ran into this past fall. The master cylinder went out on my '01 truck, so I replaced it and proceeded to bleed the brakes. The bleeder screws were frozen, had never been touched since new, never had a to do a brake job (only 65,000 miles). Tried everything, stripped the hex head nut on the dang things, would not budge. I wrapped the exposes threads with a cotton twine and soaked them in PB Blaster. Bought new vice grips with sharp edges on jaws to get a grip. Still would not move. I disconnected the brake line and drained the caliper cylinder assy. of brake fluid. I use a propane torch and heated the caliper case near the bleeder screws and finally broke free the bleeder screws. I spent parts of 2 days getting that job done.
When you put the (new) screws back on, use an anti-seize lubricant on the threads like you use on spark plug threads.
