I know the question has been asked dozens of times but there doesn't seem to be a lot of consensus on it. Here is what I have. 69 B-body, big block, manual transmission. Power brakes with disc front. Brake lines go from the master cylinder to a distribution block which houses the safety switch for the warning light. From that block there is a line which goes to the rear. Before getting to the rear brakes there is a holdoff valve or proportioning valve as some call it. The other side of my distribution block has a single line which goes up to a metering valve and the metering valve then has two lines which feed the front brakes.
I was going to plumb the line lock in the line between the distribution block and the metering valve but some tell me thats bad. My other option is to plumb it between the distribution block and the holdoff valve, but some say thats bad. Still others say after the holdoff valve with some saying never do that. In short, every version has someone who says it works and someone who says never do that. I want to hold the front brakes on without screwing up the oem valves. Is there someone with a manual transmission car who wants just front brakes on who has the SAME three valve system as me who has a working line lock in their car? If you do where did you put it and what ports did you use on the lock?
Also I dont want to take any of the three valves out of the system.

I replied on DC.COM to this, it needs to go after the switching valve, simple

My 65 swapped to frt disc and line loc.I have mine mounted by the master and going to rears Its plumbed before the PP valve.Has been there for like 15 years and no problems.Rocky

Why don't you ask the maker of the line lock? After all, they ought to know how they designed it.

Where you put it is up to you really.

If you use it in the rear brake line then you need to plumb/wire it so that it blocks pressure to the rear brakes. You will need to use your foot to put pressure on the front brakes. Safety issue here is that if you forget to release it you will not have rear brakes when you try to stop. Using this method can get tricky with a stick car.

If you use it on the front brakes you will need to plumb/wire it so that it holds pressure on the front brakes and you can use your foot for other purpose. Use the switch to release the brakes when you launch. If you forget to release the line lock it will be very obvious and you won't have a lack of brakes issue like you could have on the rears.

In either case, it needs to go after the warning light switch.

I have the same vehicle/setup as the OP. I mounted the lineloc right below the master and plumbed the line to the fronts through the lineloc.

Here are some other members setups ...

So I can put it between the distribution block and the metering valve and I am good to go.

NHRA outlaw using the line lock on the rear brakes years ago, I'm sure it is still illegal to use it that way
OP, if you install it in the brake line between the master cylinder and the brake warning switch the brake warning light will come on when you have the line lock working, which is one way of knowing it is working
I've plumed more than one car that way, most people like it

here's another really neat setup

There is some nice looking installs there, but isn't the point of installing a dual master to have redundant brakes? And if you don't have that factory type shuttle valve in there, you don't.

??? Nothing gets removed to install a lineloc !

Also, the reservoirs are dedicated to front or rear.

line lock is for drag racing/ burnouts or used as a hill hold?
mine is installed right after the master to the front valve

My point was, looking at the pictures, when you see a nice clean install, but no shuttle valve, you don't have redundant brakes.

This has come up many times before, but to prove you don't have "safety brakes" without a shuttle valve, all you gotta do is go out to your car, open a bleeder, and stomp on the brakes. The pedal will go to the floor.

If you are going to the trouble of installing a dual cylinder, go all the way. That is what I am saying. Stirring up the pot I guess, sorry.

Are we to assume you have personally done that test yourself?

I put a dual 68 Ply master on a street rod. When a front cylinder leaked I was surprised to find the pedal going to the floor. After a small amount of research, the answer was there.

Its a real simple test, nobody wants to do it as they have believed for so long.....not unlike myself.

Best of luck to you.

Not trying to be a wise guy but I have done just the opposite. I put a dual M/C on my 63 from a 68 B-body and I can open a bleeder in the front or rear and still have about half my brake pedal. Don't know why you did not because it is two separate systems. All the combination valve is for is to push a piston over to turn the brake lite on to warn the driver if one side looses fluid/pressure which I don't have in mine and the later combination valves also have the prop and metering valve in them. Like I say I don't mean to sound like a wise guy or dought your word as I can only say that I have done a few cars like my 63 and always still have brakes if one side gets a leak. Course its not a great pedal but it still has a 1/4 to maybe half pedal and still stops the car but only on one axle of course. Ron

Baloney sauce
Go look at any race car with dual master cylinders with no "shuttle valve"
Mine works fine, my new race car will have dual master cylinders with no shuttle valve or the fitting for it also

Oh, it will work fine, great actually. But if you lost a line, the pedal will go to the floor.

Hey, here is a really easy test, put the line lock on, then press the brake pedal. Try that and report back to us what happens.

No, that is not all the combination valve does.

The valve that lights up the warning light also closes off the leaky half of the system keeping a firmer pedal than otherwise.

The combination valve also provides metering, proportioning and functions as a splitter.

I'm pretty sure if none of that was needed then none of the factory engineers wouldn't have put it in there, notice how they all did it.

I think I'll listen to the engineers.

I put a line lock on the front brake side and completely removed the combination block that contains the warning switch. When I hold the line lock button down and lock the front brakes, the brake pedal gets hard as a rock with only the back brakes being actuated by the brake pedal. Pedal doesn't go to the floor. If anything, my pedal stays up higher. My 67 Caprice must be different than a street rod.

So with the line lock blocking that front brake line, and the line being already filled with fluid, there would be zero piston movement for the front brakes, yet the pedal moved enough to apply the back brakes.

That is because the flow of those two lines is connected internally, and they have to be. If there was no internal flow/connection, the pedal wouldn't move enough to apply the back brakes. This also means if there was a leak in that front line, or the rear line, the pedal would go all the way to the floor.

As state above, the switch valve shuttles to the leaking (low pressure) side to cut off flow down stream from the valve, allowing all master flow to go to the non leaking side.

This topic comes up from time to time, I'm just trying to inform folks they ain't as safe as they think they are. But, no one will open a bleeder to test, so this post will end like all the others.

Stay safe, hope for better days ahead, and a spring with a racing season to it..

Edit: It just occurred to me that most all of us have already tested this thing as I wanted to. You change out your rear, or something on the front end and you gotta bleed the brakes. So, with a guy on the bleeder, you push on the pedal and it goes to the floor, eh? You floor it a couple times, refill the master and you got it. BUT, the pedal went to the floor! Would it do that IF you had redundancy?

[so was it Billy Mays that said this??]---BUT WAIT, THERES MORE! Even if you have a shuttle valve the pedal will go to the floor when bleeding, what's up with that? That's because when you are bleeding you aren't pushing hard enough to overcome the centering springs on the shuttle valve, SOO, you only have redundancy when you press on the brakes like you are stopping the car.

OK I'm done, wish I could take my car out and my sweetie to the restaurant. Stay safe.

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