Looking for help troubleshooting which part is faulty here.

Car is a 97 civic dx. already swapped on 10.3" and milled ITR calipers up front a few months back. Just did a GSR rear disk swap, started to bleed the system then decided i better swap in my new MC and booster. 1" gsr MC and booster from a 94-97 integra gsr ABS model. (does abs vs non-abs matter?).

i do the booster/MC swap, run new lines to the mc and bench bleed the MC. Now, bleeding the calipers in the proper sequence starting at RIGHT REAR. bleed it out like 15 times bubbles here and there then no more bubbles on the last 5 bleeds. good, so i move to the FRONT LEFT. bleed out bubbles on the first few, then no more bubbles on the next bunch.

HERE"S THE IMPORTANT PART
Then i move to the LEFT REAR. my dad says "hm your pedal is WAY at the very top". he holds the pedal and tells me to open the bleeder. i open the bleeder 1/4 turn and he says "the pedal is not going down at all?". i close the bleeder we try again. same thing pedal doesn't go down. then we hear a loud POP and the pedal falls pretty easily to the floor. i close the bleeder and we try to maybe bleed it again. this time the pedal sinks fast and practically no fluid or air comes out the bleeder valve on the caliper. try two more times nothing. theres no pressure getting to the back.
I test the pedal out myself and theres a bit of resistance on the pedal, it does always come back up to the top, but you can still easily hit it to the floor.

Today i read Helms and Haynes and did some tests on the booster:
--I checked the booster vacuum line by removing it from the booster and starting the engine. the line has PLENTY of vacuum so it seems the check-valve booster line is good.
--i held the brake pedal and started the car and the pedal did sink a little. but it was also VERY VERY easy to put the pedal to the floor.
--i turned the car off and pumped the pedal a few times, it would build up to the VERY top. so basically there was no travel in the pedal at all. if i hold moderate pressure on the pedal, it did seem like it would eventually sink down a bit.
--I visually checked MC, Propvalve and four calipers, i'm not leaking any fluids and the resevior level has not gone down.

SORRY FOR THE LONG STORY.

What do you guys think is the problem? Bad MC or bad booster?

Thank you!!!
-Luke