OR, wait until you get the back one working, OR use the front brake master cylinder, but under no circumstances should petroleum products be used on brake parts. You don't necesarily need to own a "spare", there are two on most bikes.
My rear brake pedal was seized on my LTD too, when I first got it. And my 81 CSR had sat so long, I had to remove the calipers to move it from the spot , and get it to roll n my trailer.. There were rust stains in the concrete under my CSR when I bought it.
Note the rust stains in the concrete. It sat for a bit.
Take special note on the bent over Kansas License Plate. This bike was purchased in St. Louis, sitting outside for many years dead, behind a brick duplex row house type apartment building. Rather than taking that clean title and registering it after writing down ALL of his info on the back of the title AND a lien release from a bank in Kansas, THIS hoosier just bent over the expired plate so that police could not tell it was expired. No sense in spending ALL of that ~35 bucks to title it. duh.
Also note no air box, or air cleaner of any sort. Whatddya need that for? Another nut and bolt rescue.