The prefered position is after the valve, which is not only proportioning valve but it is a safety valve that will light the brake indicator light up in the dash should you have a leak in the system. when you apply the pedal, if the valve sees no pressure say in the rear brakes due to a leak in the systm, it trigger this valve to light the light indicating a brake problem. So if you hook the line lock up inbetween the master cylinder and the valve, when the line lock is applied and your foot is off the brake the dash light will come on.
Having said all that, in my old drag car (70 340 duster..now in storage) I installed it in between the m/c and the valve and it works fine. The added bonus is that you know that there is pressure being held in the front brakes when the light is on.
Plus it is asier to install there as you need less fittings etc and it is sometimes tough to get the fittings apart on the prop valve with the engine in the car