Flushing an old brake system...What's a Safe chemical to use?

Hey y'all,
In the process of freshening up the brake system on my 69' Cuda that has sat for a while and prior to reassembly I'd like to run some type of cleaner or solvent thru the lines. Was planning on disconnecting the lines at the calipers and rear brakes and introducing the cleaner followed shortly thereafter with an air blast at the MC connections.

There's also an OEM proportioning valve and Line-Loc in the system which leads me to two questions:

1) As I obviously don't want to introduce any contaminants into the system is there a preferable product to use that won't leave any harmful residue. I've got sea-foam, Gumout and plenty of brake cleaner here but just a little curious as to which would be best.

2) Also I think I can flush the lines without disconnecting at the proportioning valve and Line -Loc but can't admit to being certain about this.