Really quite simple, install a switch that ties into the rear brake line and is supplied power from the same wire that is powering the front wheel line lock. This way the front line lock will work all the time and the rear brake line lock will only work when the switch is activated.
I'm considering doing the same thing and using the switch I have for my sloppy transbrake that throws the trans into 1st and 3rd at the same time electronically. I am considering a transbrake though as well, just a line lock is a lot cheaper than a 4L80 valve body
I have not looked into a drag strep rule book, But I would hazard a guess that if having line lock on all the brakes was allowed, Someone would have offered A 2 stage unit, stage 1 burnout, stage 2 "staging at the line" .
Most vehicles can overpower the rear brakes when you are sitting still with one foot on the brake and the other through the floorboard.
If you could get enough force on the rear brakes to hold the vehicle at the line and not creep through the beams.
You could install the line lock on just the rears.
burnout, turn on line lock without pushing on the brakes first, shuts off the flow of fluid and now pushing brake pedal only applies the front brakes, do burnout. letting off the brakes and roll out, get to line and push brake pedal and then turn on line lock and let off the pedal and pray it holds the car from creeping/moving. That is allot to get right while staging and a few problems with it.
1)hoping the starter doesn't start the tree before you are set.
2) rear brakes holding the vehicle from moving.
lower power cars the rear brakes might do it. Drum brakes with the "self-energizing" characteristics may not want to fully "let go, while load is applied (foot on the gas) .
