If I replace my from calipers, do I need to completely drain the brake system (non-ABS) to refit, or can I just refill and bleed. Do I need any special tools that would make bleeding the system easier?
It's worth getting a brake pipe clamp from your local motor factor to clamp the flexible pipe - should only be about £5-6. Tghis minimises loss of fluid and you should get away with just bleeding a small amount out.
If you are due a brake fluid change anyway, why not do as above but bleed all 4 corners, taking about 200-250ml out of each corner (topping up the master cylinder as you go). Then you get the brake system filled with nice, clean fluid without too much risk of air loocks.