Proportioning for brakes is done for/by weight distribution
Pickups usually use 70/30, 70% of braking power goes to front wheels, 30% to rear wheels, because bed on an empty pickup is much lighter than the front where the engine lives
Cars and SUVs used 60/40
Some mid or rear engine vehicles use 50/50, it all about the weight distribution
If your master cylinder has one brake line coming out then it will run directly to a Proportioning valve, and it will have 2 lines coming out, one for front and one for the rear brakes, these do come in adjustable models
If you have 2 lines coming out of Master cylinder then it has internal proportioning, line farther forward is for rear brakes, line farther back for front brakes, so its reversed.
If you follow the rear brake line down it will run to a restrictor or ABS valve, which acts as a proportioning valve, its usually on the frame rail
You can replace this with an adjustable proportioning valve, and then go out to a gravel road and slam on the brakes a few times and adjust the valve as you see fit