For line fittings, there are two standard thread/diameters. One for 3/16" line and one for 1/4" line.
But do be aware; there is a long style and a short style of each one.
1. A1-3 is for the 3/16" tubing. It measures 3/8" by 24 unf.
2. A2-4 is for 1/4" tubing. It measures 7/16" by 24 tpi (threads per inch).
There is however, another series of tubing nuts/fittings that use larger OD's. These are called "master cylinder" nuts. As the name implies, they were originally used to join standard 3/16" and 1/4" tubing to the master cylinder.
There are three m/c nut sizes available for 3/16" tubing:
1. 7/16" x 24 tpi
2. 1/2" x 20 unf
3. 9/16" x 18 unf.
For 1/4" line there are two sizes of m/c nuts:
1. 1/2" x 20 unf
2. 9/16" x 18 unf.
Over time these nuts began to be used elsewhere in the brake piping system. Now you never know where they will pop up. On your truck for example, in addition to the MC, you will find them used to connect the supply and outlet lines to the proportioning valve:
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They might also be used on the load compensating valve or elsewhere in the rear hydraulic circuit. I don't think they are and you shouldn't have to be concerned about them - but since I am not really sure what your exact repair intention is, I just thought I would mention their existence.
Trust me on this, the best way to be sure you get the correct fittings you need (with just one trip to the parts store) is to bring the old fitting with you.