1970 buick electra disc drumrear brake line routing

my 1970 buick electra appears to have rusted through on the rear brake line, way at the back, just as it loops around mid left right and connects to the rubber line that then heads to a splitter. anyone else have this happen? it seems that the rear line comes off the master cylinder and into the distribution valve (attached to the frame) and then back out and down the drivers side of the car. just at about the level of the bracket that holds the transmission shift linkage it looks like it enters a proportioning valve, and then comes back out and heads down towards the rear. is there an easy way to disconnect the line from the proportioning valve? seems line the bracket holding the trans selector arm blocks easy access to the line nut connecting the rear line to the valve. any suggestions?

the bracket is just a couple of bolts, yes?

it's not often you have a situation on these old cars where you have to disassemble X in order to access Z but it does happen on occasion.

1966-1970 is also all the same generation of Riviera and we do have the 1966 Chassis Manual so it's possible this may also be helpful:
https://www.teambuick.com/reference/...ssis/index.php