Right then so it's a gradual thing, not such that you'll be driving gently down the road, slowly applying the brakes then all of a sudden the ABS takes over and applies the brakes at 100% out of nowhere, which I was worried since that's a much more difficult condition to diagnose.
Yes this is a common problem on all old cars not just these, lack of 24 monthly (no one seems to bother doing preventative medicine on their brakes and I cannot understand why) brake fluid changes lets more moisture into the fluid rusting out the calipers and caliper pistons to themselves causing them to not move smoothly and bind up
The proper repair for this is to take the vehicle off the road and totally rebuild all four calipers professionally, replacing the rubber piston seals, piston boots and change the old rubber brake lines to stainless steel. You need to inspect all pistons and calipers for damage from rust and you need to clean all of the internal surfaces of each caliper bore and piston. If you don't, you'll keep having this problem until you do a full rebuild