I MIG welded that notch and filed the spot weld down flush. Been fine for about the last eight years or so. Weld it hot, on a high setting and burn the weld in good. Clean it down to bare shinny metal before welding. That knuckle on the early models is low carbon cast and welds OK, it is almost steel and not like black cast.
Various reasons for the brakes to lock up like that. If the caliper is sticking on the mounting bolts (pins) the pads and caliper may overheat, it should be free floating. They are stiff when trying to move the caliper on the pins, but the caliper has to move. A sure tip off is when one pad is worn almost completely and the other pad still has meat on it. Check out the mounting holes on the caliper and make sure the bushing isn't twisted up in there and/or full of crap. I'm not sure if the 91 has the Teflon sleeve inside the caliper mounting bushing or not, if it does it may be knotted up in there jamming the caliper on the pins.
People have been known to bend the caliper mounting bolts (pins) or cross thread them. Which has basically the same outcome as the caliper being stuck/jammed on the pins and not moving. The inner pad rubs too much and makes excessive heat.
The calipers sometimes get rusty in cylinder bores on the inside from bad brake fluid (water) and/or the dust seal rips and mud or whatnot gets between the piston and the bore from the outside (happened to me, locked the caliper down solid).
The rubber brake lines can separate on the inner layers and it can keep the piston from relaxing when the pedal is released.
Not a bad idea to purge all the old brake fluid out, there may be moisture in there. Moisture mixed in with the fluid corrodes the inside of the brake system and can even turn to steam, which may be another reason for the brakes locking up on you.
Those pads can get really hot, the heat transfers to the caliper. I've had bonded pads get so hot the bonding (glue) let loose and the pad separated from the base. I had one pad separate from the base and fall out onto my driveway, shortly after a hundred mile an hour bonsai run up the autobahn. I heard the pad go "tink" on the inside of the rim as I was walking away, I went back for a look see and noticed the pad laying there. That could have gotten real interesting backing out of the driveway the next morning if I hadn't noticed.
