I'll weigh in:
I think these are cool for cars with no park brake. For use on the move, I think they are a bad idea.
Firstly, they aren't a one-way valve so you can't have "another bite" you have to get brake pressure right and then crank the lever. If I drive up to something, hit the brakes, line lock it, and the car creeps, I can just pump the pedal until it stays put.
You have to plumb the brake lines to somewhere you can easily reach the lever. This might make bleeding a PITA.
You don't have fingertip contra. You have to yeah to wherever the lever is and turn it. I have a momentary switch on the gear lever on piggles, and a constant switch. I can park the car on the constant switch, get in, put it in gear, clutch, then, with one finger on the momentary switch, turn off the constant switch - the car doesn't move. A millimetre. Then bring the clutch in and when I feel it load up, lift my finger and the car is away without any rollback.
I"m not saying that's impossible with a manual valve, I just think it's much harder.
And in any case, an electric line lock is cheaper.
http://www.lowrangeoffroad.com/index.ph ... k-kit.html
I think in reality these are two different tools for different jobs. I love my line lock.
Steve.