I think you should make sure your pressure differential valve is centered. Here is the procedure to do it
. And yes, the front and rear are totally separated and, in theory
, you shouldn't have to bleed the rear if you only changed a part in the front.
What I think happen with your situation
You probably have the cast iron combination valve. Normally, you should have the brass version as the cast iron one is only listed for 71-72 GM trucks
, but I know one member here had one on an El Camino 71 and the one below was found on an 87 Monte Carlo SS
(and was replaced by the one from the right found on another Monte Carlo SS of the same year). Does the workers on the assembly line take one model when they ran out of the other or was it replaced afterward by a «careless» mechanic? I don't know. But, anyway, they work the same.
The reason I'm telling you this is because the valve stem doesn't work the same on both. For the brass combo valve, it has to be pushed while bleeding the front brakes like it says on this figure from a 72 Buick chassis service manual:
But with the cast iron combo valve, it has to be pulled
. The next figure show 2 special tools that can be used to do so:
So if you have the cast iron version and you pushed on it while bleeding, you actually closed the valve, hence why you got the hard pedal and no fluid flow. The difference between the two valve stems is that the one that needs to be pulled have a little knob at the tip for the tool to hold it.
Furthermore, by having a leak in the front brakes, you tripped your pressure differential valve which partially blocked your front brake lines, thus why you couldn't bleed the front at first.
Then, I suspect your rear brakes are not in perfect shape as well. I would check for a small leak (look also behind the dust boots of the wheel cylinders, there shouldn't be any fluid passing by the cups) because what I suspect happened is that by blocking your metering valve (the valve stem) while bleeding the front, the pressure increase in the front was larger than for the rear and the pressure differential valve went on the other side! Then you went to bleed the rear, which was now blocked off the same way the front was before!