Workin on a 67 Belvedere. I have a morose line lock I want to install. My question is, can I install it between the master cylinder and the proportion valve, or does it have to go after.

before the proportioning/distribution block

Before the distribution block. Will it work if I put it there?

Yes.

The rule book says: "If brake system includes a differential pressure switch, line-loc installed on front brakes must have solenoid installed after the differential switch." I'm pretty sure your 67 has such a switch.

If your not racing a NHRA class then their rule book does not apply, correct One of my old NHRA legal drag race cars had the line lock on the inner fender well between the dual M.C. and the distribution block on the frame, it went through tech a bunch of times at both NHRA National and divisional events that way. The brake light would get bright telling me that the line lock was working fine

Mine is mounted between the master cylinder and the distribution block. It’s worked fine for years and has been through tech probably 100 times, never questioned.

I agree it may never be checked and it will work but that doesn't make it right. Years ago I did some checking into that rule and was finally convinced it made sense. The result was I removed the dist block totally. I've slept a couple of times since then and can't remember all the details. I do remember I had a hard time even finding out what a differential switch was.

Typically it's just a mechanical switch that has the front and rear brakes plumbed through it with a passage between the two with a shuttle valve in it.(or on some later model stuff, it's split diagonally) If pressure is lost on one side while applying the brakes, due to a failed hose, line, etc., the shuttle is supposed to be pushed to the side that has lost pressure and shut it off to prevent total loss of braking. It usually also has an electric switch in it to turn on the brake warning light if actuated.

Installing the front brake line-loc between the MC and the differential switch will cause it to flip and block the rears. Usually you can stab the brakes and get the switch to go back to neutral. Sometimes that doesn't work and other methods have to be used.

Some people pull that stuff off when using any aftermarket or non-OE brake components. Some don't.

I was just wondering the reason for this a couple weeks ago. I need to do a new brake line across the firewall while I have the engine out and part of me wanted to buy a fancy prebent stainless line. I was curious if it's really necessary to follow the instructions, blocking off one outlet and T off the line lock. I guess I'll just follow the instructions.