my car is originally an A/C car but I'm building it without A/C.So the car originally has the small valve on the brake line under the drivers door.Can I leave this valve on?It's front disc and rear drum.

You can leave it in place. It reduces rear pressure for weight transferrence.

If you have a 67 with single piston fronts, and a sb engine w/o a/c, you could probably up the rear cylinder size to 15/16 with that valve in place, esp if you are running manual brakes.

Are you referring to the cylinder in the rear drum? I do have single piston fronts with a sbc.

The valve has a plug, remove the plug and internal parts, replace the plug.
David

Yeah, I was talkin' about the rear drum cylinder. Single piston fronts and a 15/16 rear with a valve was what the late 70s Fbodies ran. Their valve may have had a more aggressive slope though.

But you could disable the valve like Dave said. That would be easier, and probably a better idea. It wouldn't really hurt just to leave it alone either. There are alot of factors involved with weight transfer and balance, including ride height.

If you bypassed it with a line, I bet someone would like to own an intact valve.

when I replaced all my lines I got a 2 piece front to rear so I installed the valve.