Thanks for the replies! I have looked at a number of options but I was hoping someone had one leftover after doing a dual master conversion. I was hoping to keep it all original despite how pathetic the system was.
my old volvo has a thing that looks like a cylinder with a heavy spring behind it and it has discs on the front but drums on the back. I think it takes up a bit of the fluid pressure to allow the back brakes to expand before the front discs grab... or something. I'd call it a hydraulic pressure accumulator. that thing on my old volvo is plumbed into the brake likes and up under the floor and one of the few things Ive never had to mess with.
likewise my 88 ford van has rear drums and front discs and it uses a device that is connected to the master cylinder and resides on one of the brake lines. it looks like an extra fitting where the line connects to the master cylinder. . I had to replace it because it started leaking otherwise I may have just thought it was a plumbing fitting. I dont know if a mustang ever had a similar device but I can see how it could get eliminated during mods with out really considering what it was meant for. someone else here may be able to comment on why it was used. I believe it's to help the brakes grab more evenly upon a panic stop. If it were eliminated you would still have brakes. I drove my van for a while with the thing basically disabled because I did some stuff to it to stop it from leaking while I waited for the new part. I didn't feel much difference but I put it back to original once I got a new part.
if you have single circuit brakes some add a float switch from a newer vehicle, they are used to provide a low level indicator and can be adapted. some like to add a low level warning buzzer to keep things looking original on the dashboard, and provide a bit more safety with a single circuit system. that way it'll warn you if you get a leak and use up too much fluid before you get to the failed brakes stage.
the brake distribution block may have a piston that can shift to either side and it may have a warning light so if you loose one side of the brakeing system then the spool shifts, no fluid is lost but the brake warning light comes on. it might be possible to use one of those so you get some warnign if you loose fluid from one wheel. It might block fluid off that is being sent to a leaky wheel so you can retain some fluid in the system to stop..someone else might explain that better. . if that distribution block just has three lines, then perhaps one whit a switch for a brake failure warning light can be added if its not like that now.