'72 K20 Braking variations

[72-c20]

stock frontdisks, drums rear.
was braking fine, but needed a bit more bleeding...

after I bleeding, the back wheels would lock up really easily. if you pushed slowly slow, no lockups.

then it switched tonight. harder pedal than before. like no boost at all. thought I wouldn't stop in time a couple of times! and I could head a sound like the pressure bleeding off coming from under the hood....after I let off the brakes

then on my way home, on about the 3rd time stomping on the brakes, I got 'em to lockup again, and now it's back to the quick lockup again.

I've searched through all the brake problems, and didn't see one where it went back and forth....

depending on whichpost I read, I lean towards the booster, then lean towards the check valve.

thoughts?