The existing lines look corroded from the outside, but I think they may be revivable on the inside since they haven't been taken apart for some time, they have somewhat clear dripping fluid, and they've probably been in place since they were working last with whoever the last owner was...who knows how many years ago (20? 30+?). In other words, its in stock-ish condition, not a gutted van (it used to have working brakes before it just sat for decades)
After replacing the rear brake shoes and wheel cylinders I continued on to see if I could get the lines to the rear brakes to flow with pressure and engage the wheel cylinders to the shoes. The fluid was dirty in the master cylinder and removed what I could in the master cylinder chamber and replaced it with new fluid. Then I proceeded to bleed the brakes from the back with the "buddy system" pump and bleed method. It worked at first with pressure coming through the bleed, with fluid coming through but then stopped and now there's no pressure and flow in the line. I may have-- but not sure-- if either some dirt contamination or air got in the line, but its no longer flowing with pressure at the moment. I was able to drain through and replace the fluid in the master cylinder a couple times before it stopped flowing. I've decided I'm gonna go ahead and replace the front brake pads and calipers next before worrying about getting the pressure going to the back, and work on the line flow and pressure last once I now I have clean and new parts on all four wheels. The front brakes need replacement regardless.
My question out to you is, is there a suggested method to use some kind of pneumatic vacuum tool that I can use to force flush my lines, from any of the front or back wheel bleeders on any of the wheels, to and fro? I'm ok with investing in something that I can hook up to my compressor and use to force feed and flush the lines through the whole system if its not too expensive. And I may go ahead and replace my master cylinder while doing so, but I'd like to get the one in place working if possible with just the lines flushed and flowing.
Or is there another method you'd suggest all together to get the lines flowing with the existing master cylinder in some other way?
This will be a last ditch effort before I decide to replace all the solid lines (I think there are 8 of them). And I'd also like to try and use my existing master cylinder, to just get what I have working if possible.