On my 79 malibu I just put a new rear with disc brakes factory gm metric calipers front and back has manual brakes and factory manual 7/8 bore master cyl while changing rear I lost all of the fluid out of my master cyl going to rear brakes I got everything back together and started bleeding brakes after several hours of bleeding I still cannot get a brake pedal the issue seems to totally be rear brakes you can pump brakes up loosen bleeder on front brakes and shoot fluid across building but when you try to bleed rears very little pressure if you ever get any pedal at all it's less that 1/4 way up from floor and if the rear bleeders are opened you lose it all after first time .... I pushed little button on the end of factory prop valve no help I've bled the master I've put containers at both rear wheels with fluid in them with plastic hose going from bleeder into fluid worked pedal back and forth and get a lil air doing that I left it set for 2 hours gravity bleeding refilling master every 30 mins so it didn't go dry and still pedal is only 1-2 inches off floor and very spongy .... I did put a adjustible prop valve to rear but removed it trying to get this thing figured out .... Any thoughts ?