That could very well be the case. If one of those springs is not installed exactly right, it does not unlock. Let us not ask how it is that i know that.
Not to be difficult, just different. I put Toyota front discs on the Pig in 2004 or so. Kept the stock '74 master cylinder. Had great brakes. Added Posers rear discs a few years later. Stock '74 master. No proportioning valve. (But longer wheelbase). Still great brakes. Years later added the...