All I know is: with the lineloc mounted right at the master and in the line to the front brakes it will hold the fronts on rock solid (with no rears) for as long as the solenoid is energized. I don't recall but I don't think the "brake" light comes on - but there is a seperate lineloc light to show its on (just in case you forget you're holding the button I guess!)

If you read the rest of my post you would see it says all later valves had the prop valve and metering valve in them but the early ones did not. And that pistons does not block fluid from the other side. It is two independent systems. Look at the inside of a dual M/C and you can see they are independent of each other. Yea you will lose half the pedal if one side springs a leak but if you look at the inside of the M/C you can see M/C piston will compress the spring in it on one side and still have the other side with a much lower pedal. I did open a brake line on my car and I still have half my brakes. In fact I had both of my rear bleeders open and pumped my pedal and still have just under a half pedal and front brakes that stopped the car but not as good as all four of course. Ron

The safety valve does not block fluid its only there to turn the brake light on the dash on to warn the driver. If you think about it when you get a leak in one side it loses pressure and the other side of the M/C that still has pressure pushes the switch valve over. Now if you do not have the safety valve between the two brake systems and you get a leak in one side the other side that normally pushes the switch valve over still has pressure and it will still have pressure wether it pushes a valve over of its a solid line with no valve and not connected to the other side at all. In fact the early systems on some vehichles did not have a switch valve in them and they also had no metering valve because the first one were drum brakes which don't use a metering valve and the prop valve was in the rear line only when it had one on the early systems. Ron

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I agree 100%
that's what I remember ….lineloc mounted on firewall with line out of master directly to solenoid then to the low pressure switch where lines go to frt and rear wheels also separate light to indicate system in use as long as you manually hold button on.

While the topic of line locks is being discussed, I submit this question.

I have installed my line lock as mentioned directly in the front line, before the pressure differential switch and not only did it not work, it caused the BRAKE light to turn on. (Please forgive the terrible photo editing to add the line lock solenoid)

I then decided to read the instructions, and installed it in the line AFTER the pressure differential switch. This worked better, didn't turn on the BRAKE light, HOWEVER after about 2 seconds the truck will begin to roll just at idle. I do not know if this is a faulty line lock solenoid, a bad plunger in the solenoid, or what. But it definitely will not hold pressure to the front brakes. My hope is that its just a bad solenoid and once I replace it I will be able to hold the front tires. The solenoid is staying powered, but fluid seems to be moving through the unit.

SO the question, Can installing it as shown by HURST cause my front wheel roll problem?

https://documents.holley.com/1745000_instructions.pdf
link it to the HURST instructions.

Did you buy new lines to use on the car for this or did you cut and flare the old line or use feral type fittings for sealing the old lines? If either of the latter two try tightening them up some more and retest Can you see fluid leaking out anywhere now after using the line loc?
Let us know what you did to fix this and the results

In my installation the lines were all new. There are no leaks, system was properly bleed, and the brakes work perfectly fine in normal driving, pedal holds firm, stops on a dime.

Only issue is when using the line lock.

The line lock is not holding the line pressure evidently when applied

My 76 Volare has had the line lock right out of the master port for 30 years. Never an issue.
Doug