depends where you put the line lock. if you put it right after the mastercylinder and prior to the proportioning valve then the pressure differential is going to throw the safety valve thats ment to be there in the event of a hydraulic system failure. tap into the line post proportioning valve and i think you'll be good to go. i tried a line locker b4... never worked; i may have installed it improperly idk. long story short i yanked it and sold it. manual proportioning valve if you really wanna have fun. this is the route i was planning on going later down the road

 

If you just want to temporarily disable your rear brakes, just use a small set of vise grips on he soft line from the frame to the rear axle.
Or the smart way might be to put a couple of tire chocks in front of the front tires. Power brake for a few seconds, nd then you should be able to let off the brakes indefinitely.

 

that's fine and dandy but creeping sideways is much more fun than sitting in one place layin rubber

 

Like said before it needs to go after the prop valve so the hyd system thinks everything is fine.