So I have a mid 70's 50lb Darton Magnum that has broke 2 strings so far this year. The first time it was last years string, figured it was just old. So I made a new one a couple of months ago and it broke too, 12 strand flemish B55.

After the second break I checked it for burrs or rough areas, didn't really find much, but sanded it down a little anyways.

My question is could brace height being too tall cause this? I found an old Darton catalog which states BH should be 8 3/4" to 9 1/4", and I had it tuned around 9".

I made another string yesterday, 14 strand, and the BH settled into 8 1/4" and I think I'm just gonna roll with a lower BH for now. Really just want to see if anyone else has had issues with these low AMO bows busting strings at the knock?

I had a Bear Ausable that had the same problem. One of the corners in the string groove was really severe, so I rounded it down with some sandpaper and that fixed it. Also, if your arrows are too light it may be rough on the string

Brace height is not a concern. If the string is breaking in the loop there is something abrading it. You probably have it taken care of now. It doesn't take much just a a piece of grit or bubble in the finish.>>>---> Ken

Not the brace height.  Some spots are so minor you won't feel or see them, but they cut strings.

Also, you could pad the loops but you probably already thought of that.

Run an old pair of pantie hose or similar material thru string grooves an see where it catches. Got to be a rough spot somewhere in the string grooves.

I have made padded strings before, but that just delays the inevitable.

I used a cotton swab when looking for the defect, found nothing, so maybe pantie hose would be a better option, never thought of using that.

another thing people dont realize keep super glue away from any string, specially the ff material it will melt that stuff instantly creating a hard spot
even the fumes will cause problems, yours sounds like a fiberglass issue, maybe cracked and only split open when it is drawn had that happen

I had a similar issue, on the top loop. I tried to smooth the tip of the limb a couple of times with no success. Eventually I asked a cobbler to cover both tips with a small leather hood. That worked. I've had them in place for 6 or 7 years by now, they didn't give me any problem.
Hope this helps.
Max

Sorry, I've just read your question again, you're breaking at the nock, right? As some said before me, 1) it's probably the nocks too narrow. Do you use a serving on your strings? 2) could be  superglue, if you put it on the serving/nocking point knots

I have told this story before; I was shooting 2114s out of a 70# bighorn and breaking B-50 strings at the top nock right and left.

I asked for help on the old Stickbow site, Dan Quillian said "you are dry firing your bow with the light 2114s, this is causing your strings to break".

I thought, "you old goat what do you know" and kept shooting my 2114s for a while and breaking strings. I was getting 205fps out of the bow.

That year for hunting I changed to a heavier 2018 that was about 100 gr heavier than the 2114s, I never broke another string.

Old Dan knew what he was talking about and I had to eat a major helping of crow.

I have a PSE Heritage series longbow which wore thru strings rather quickly.  I took to wrapping the loop with b-50, much like serving an endless loop.  May have slowed it down a foot or two but I would get a year plus out of a string rather than a week.

I'm flinging some ICS hawks at around 450 grains, so I would hope that's heavy enough.