69 1/2 Six PAck Bee

I have a '90 SR5 4x4 with the rear ABS and has probably the worst brakes of any vehicle I have ever possesed. I am just going to cut to the bottom line here. Even though this truck has rear ABS the master cylinders that I can buy through the local parts houses only list them for 92 and up w/ABS. What is the difference between the 1" bore that is listed for my truck and the later years' master cylinder?

Thanx,
Bud