87 Lesabre Brakes don't bleed after self install of the master cyl

I bench bleed the MC and installed it on the booster and could not get the right rear and left front to bleed.
I read how there may be a safety feature in the MC that provides protection in the event of line failure. You were to select a pair of wheels, either both front or both rear, open the bleeders and give the pedal a slam to reset the safety feature. It seemed to work when the front bleeders were released but by the time I got to the rears the right rear still failed to even let out a burp.
In the old days you could just slao a bench bled MC and start at the farthest wheel bleeder and bleed away.
I just can't figure out what I'm missing here. Sure could use some guidance here.

Bumped

How many times did you slam on the brakes when you were doing the rears? I had to do mine about 3 to 4 times at least to get something to come out. If that still doesn't work, I'd check all connections in the brake line from the MC to the back wheel cylinder. Maybe there is a defect in the MC itself or something wrong with the wheel cylinder on that side .

"97 VO,
Seems like we hit the brake pedal about 2-3 times then I thought too much fluid was being pumped through the left rear (which always seems to flow). Only the right front and left rear are the ones that always bleed.
If it wasn't so frikkin' cold here (unheated garage) I probably would have more patience.
BTW I'm working on the 2nd master cylinder. The 1st one did the same diagonally opposed lock up so it was returned.
Here are some questions;
Once the master cylinder safety mechanism is reset and both rear bleeders flow nicely, what keeps the system from thinking there is a brake failure once you try to bleed the right rear again?
Once mc is equilibrated, what type of pump strokes are used, slow and full or hard and fast?
I'll keep trying with continued repeated attempts til we get it.
FOKYL

"97 VO,

BTW I'm working on the 2nd master cylinder. The 1st one did the same diagonally opposed lock up so it was returned.
Here are some questions;
Once the master cylinder safety mechanism is reset and both rear bleeders flow nicely, what keeps the system from thinking there is a brake failure once you try to bleed the right rear again?
Once mc is equilibrated, what type of pump strokes are used, slow and full or hard and fast?
I'll keep trying with continued repeated attempts til we get it.
FOKYL
That's a good question, I'm not sure. I will ask some mechanic friends of mine for an answer. I would think that once it's reset, the master cylinder might be able to differentiate between bleeding the brakes and a simple brake leak. Although repeatedly attempting to bleed the brakes at one fitting might re-trigger the safety mechanism. After we reset our m.c. I used slow and full strokes to finish bleeding my brakes. To me, that insures that all of the air is "bled" out . It might be a matter of personal opinion though, as some would prefer to do the "hard and fast " pump strokes. Try both and do whatever is comfortable to you, I'm not sure if there are any other benefits in choosing one over the other.