line lock install - before or after Prop valve?

Ed Cannon

Pro Stocker
Jul 17, 2009
Johnson City Tn
Killerdave8813 said:
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!!!
Mine light and pedal does the same thing. but if my rears are free mine is staying that way. Like Slimbo said the front and rear brake circuits are independant of each other mostly for safety, say if you were to blow a line in the rear you would still have front brakes and vice versa.


Pro Stocker
Feb 7, 2005
Philly Area
I am not an expert on proportioning valves by any stretch of the imagination, but here's what I DO know:

The "proportioning valve" used on zillions of GM cars is actually 2 things in one. Obviously, it's a proprtioning valve to reduce pressure to the rear brakes when you dig deep into the pedal, preventing the rears from locking up prematurely as weight transfers off of them to the front of the car. That part I think we can all understand pretty easy. (Although, I am as clueless as the next guy as to how this little miracle is achieved. In all honestly, if anyone's got a link showing how it's done I'd love to learn!)

But to the point at hand....

The OTHER thing that little chunk of metal does is act as a Pressure Differential Sensor. A pressure differential sensor is nothing more than something that measures pressure from both of the lines coming into it from the master cylinder. If everything is working correctly they should be the same pressure ON THE INPUT SIDE. If they are different it moves a piston that can contact an electrical trigger on either side, which grounds the "brake" idiot light circuit and lights the light on your dash. It can move in EITHER direction- front pressure higher than rear lights the light, rear pressure higher than front also lights the light. Like a piston with brake fluid chambers on either side of it. If one chamber is higher pressure than the other it moves the piston over, makes an electrical contact and lights the "brake" light on the dash. Please note this is on the INPUT side of the valve- WHAT PRESSURE IS COMING FROM THE MASTER CYLINDER DOWN BOTH LINES. It has nothing to do with what final pressure (either the same or different) is actually going to the front/rear brakes.

So if you think about it, installing the line lock solenoid either before or after the "proportioning valve" really makes very little difference, except that it will instantly light the idiot light on the dash when you set the line lock and then let off the brake if it's installed BEFORE the proportioning valve. The only other SLIGHT difference it makes is that when you go for the next brake application some amount of fluid must be used to move the piston in the pressure differential sensor back to it's "center" position to turn the idiot light back off. This probably accounts for the slight "spongy" feeling in the pedal upon next application. Thereafter everything should feel normal at the pedal.

Recommendation: put it after the proprtioning valve. But if you don't, don't jump off a bridge or anything becuase of that.

The annoyance of putting it after the valve is that the proprtioning valve usually has 2 separate lines directly off of it for each of the front brake calipers. That means you have to do more work to block one of them at the proportioning valve and then put a separate T in the line after the line lock. Just more plumbing to deal with. New2malibu's picture, above, is awesome for demonstrationg this- you can even see the T he had to put in after the line lick down on the frame rail.


Pro Stocker
May 24, 2007
Reno, NV


Dragway Regular
Mar 15, 2008
Pittsburgh, PA and Philly Area
Damon hit the nail on the head guys. You can put it before the prop valve, that's where I have mine. However, if I did it again, I would put it after the prop valve so the pressure differential sensor won't kick every time I am in the burnout box. I run an aluminum GM style disc/disc prop valve with factory front discs and rear disc conversions. Gear

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