testing brix

I have a few questions.

How do I get the juice out of brocolli and other veggies like cauliflower, asparagus etc. I have tried using pliers and all I get is mashed/pulverized veggies. Maybe I am doing something wrong?
Also how would I juice a lettuce leaf? One brix link from the moreless website suggested crushing the leaf and then laying on the refractometer;
I tried this and it did not work at all. Also it was suggested that cutting 1/16" of a veggie and then laying it on the refractometer would also work, well this did not work for me either.

How about beets? I put a few drops on my refractometer and all I saw was red.

Another mystery to me-I tested an onion it was 11, I then juiced this onion with a small beet then tested this beet/onion juice and got a 5. Can you help me understand why this brix was so low?

Should I assume that all fruits/veggies with low brix numbers and non-fuzzy lines (clear straight line) have NPN?

I have gone through the links listed on the moreless website, does anyone have any further readings?

thanks much!