dude, 4BBodies is probably totally right. barring a blockage or gremlins, it's your metering valve. not only can a frozen plunger do it, but there are a series of rubber o-rings inside, if one has been distorted or torn, fluid will leak past when you press the pedal, the plunger will move into rear brake failure position (in your case), lock the line to the rear and you'll have front brakes only which will also result in that incomplete pedal depression you are describing. SO, unless you're in the mood for a poor man's line lock and or a lot of continued head-scratching, get a new metering valve. Service replacements are still available that look identical to originals minus part numbers, I'd tell you where I got my last one but my reciepts and stuff are on a different continent.

I do not believe the problem lies in your pushrod, I am fairly certain they are the same. If it was a little too short you would still get SOME fluid out of the system from either the front or the rear when you pressed the pedal, the depth of travel doesn't make a difference in the pumping of fluid to either half of the system, if it did you would have uneven braking and lose control.

the metering valve is not a bad part to replace anyhow as none of this old crap (said with love) has an infinite life span and brakes are moderately important.
Good Luck!
-Andy