Mine is before the prop valve as It was easier to do that way,as far as I can tell it doesn't lock up the back brakes as I was doing some line lock
testing (burnoffs) In my driveway the other day and that seemed to work ok,the only weird thing that it does is make the brake warning light come on untill after the line lock is released and you hit the brake pedal,the reason it's doing this is because the prop valve also has a low pressure warning sender in it in case you blow a brake line or something,so when you key the line lock and let off the brake pedal it's showing low pressure on the rear side of the sender and turning the light on,also when you do hit the brake pedal mine at least has a bit more travel till the light goes off,I'll admit I never read the instructions untill after I installed it but you should always go by manufacturer instructions if you want things to be right,I'll be changing mine sometime soon and will probably also hook up the auxillary brake light switch so when the line lock is activated the brake lights are on,shouldn't be a big deal,also the NHRA rulebook calls for the line lock to be after the prop valve,I read that on here somewhere is why I started researching the situation,anyway that's my opinion!!!
Later,KillerDave