I'm finally getting around to attempting to plate the hard lines for fuel, brake, clutch. As I take these out I just noticed the lines for the master cylinder to proportioning valve are backwards. I'm not sure why the PO did this, was there even a purpose or simply didn't know the correct placement? A little background to add. When I first got this car (its an early 71) a few years ago I was as newbie as you could be. I brought it to someone in the community that I trusted to sort out a bunch of things, one being really squishy brakes without full stopping power. He did the bleeding and typical check of the brake system yet we couldn't figure out why the brakes wouldn't firm all the way up. What he finally realized, and I may not describe this accurately, was that the booster and the cylinder weren't "matched" correctly as he put it. The booster was correct for the car but he said the PO sourced a late 72/73 cylinder and the "adjustment pin" that mated them wasn't long enough to engage the cylinder fully. I saw him machine some type of extension pin, dropped it in and everything was perfect. I've left it alone since and the brakes work perfectly. I'm wondering if the PO routed the lines like this on purpose for some reason rather than fixing the issue he caused by using mismatched parts? Maybe the adjustment pin wasn't long enough to engage the front brakes and so he swapped them to engage first? I don't even know if that's how it works but he must have done something on purpose. My question now is, if I plate these or source replacements to plate should I leave well enough alone or should I "correct" this and route them properly?

Edited February 22Feb 22 by Richie G
photo issue

Here are some old threads about it. Captain Obvious makes some detailed points about possible performance differences and problems in the second thread. You might find the reason that the PO switched the plumbing in the threads. But, really, the main difference seems to be the size of the reservoir. The disc brakes move more volume and the level probably drops faster as the pads wear. Probably could just swap the reservoirs and be fine as far as braking performance and maintenance. Choice seems to depend more on what the goal is. Points deduction at the car show.

https://www.classiczcars.com/forums/topic/21826-brake-master-cylinder-240z/

https://www.classiczcars.com/forums/topic/66168-brake-master-cylinder-identification/

Thank you @Zed Head very informative. I'm guessing the PO, who it seemed was just flipping this car, wasn't all that concerned about performance as the brakes barely worked well in the first place. I'm guessing he sourced this MC which wasn't from the original car and just hand mangled the brake lines to match the designation F / R since that's what it stated on the MC. Here's my uneducated take on this. It seems from these posts there was a progression over the years where F started in the front with the larger reservoir, migrated to the back in "gen 2" but the MC still had a smaller reservoir for the R, then "gen 3" left the positioning but upgraded the R to a similar sized reservoir which is what I have now (maybe coinciding with rear discs). As for the @Captain Obvious observation on release, wouldn't that situation affect the brakes regardless of the check valve? Meaning if i had air sucked in it would be happening already just on the other set of brakes? The size of the reservoir for F/R makes sense to me, but the check valve one might be going over my head.