A hissing sound when you push the pedal down is likely a leaking booster. If it was about to fail, but you had a firmer pedal before doing the work, it might well have failed when the pedal sank lower and hit a new spot. You may also have air in the master cylinder and need to do a more thorough bleed. It's also possible, if the whole system is old, that the master cylinder has now gone bad. These things often occur together, when parts are disturbed, or moved into positions they're not used to. I'd start with making sure that the rear brakes are adjusted properly, then a very thorough bleed of the master cylinder (if it went dry during the work, you may even need to bench disconnect the lines at the MC and bench bleed it first), to see if you can get the pedal height up. If you can get a firm pedal, then you need to address the possible vacuum leak in the booster. If you just can't get a firm pedal, or if the pedal sinks under steady pressure, look to the master cylinder, but if you just did a bunch of work, you should double check adjustment and bleeding first before suspecting the MC.
