Question on master cylinder

Depends on the MC you use. You might be referring to the 80's Dodge truck one, which I think is the basis for Dr Diff's. I used a 97 Breeze ABS MC. Both ports are the same size - 3/16" tubing w/ inverted flare and I recall the smallest tube nuts for that size tube. 1970's Dodge cars used different size tube nuts for F & R ports so you couldn't mix them up, but they might have dropped that by 1980's. Be careful of MC's later than ~2000 since many changed to "bubble flare" ports, w/ metric tubing (6 mm, I recall, though is actually close enough to 3/16" to interchange).