I just had this problem. I am not sure if this is the right way but it is the way I did it. Start at the closest fitting to the front of the car and see which is most likely the front end fitting of the rear brake flex hose, (unless you have something like my 74 manta parts car that had a fitting near the bottom of the bellhousing). Since you are going to be bleeding them anyway pump em after you open the fitting and see if the fluid comes out. If it pumped fluid close it up and go to the next fitting and repeat the process till you locate the blocked area. When you do, remove the blocked area section of hose, in my case a rear flex line, and replace it. Bleed brakes and you should be ok.
hopefully someone more qualified than me know a better way and will post it and we can both learn....
c