Something definitely wasn't right there, with your old kit - I have OEM front and rear brakes and they lock (to the point of ABS intervention) perfectly.
The brake hardlines are made of cheese, even here with nil corrosion they want to twist as you experienced. I found penetrant, good blast with air, more penetrant and an initial twist in the tightening direction, helped. Also, hanging on to the flex's hex with a 17mm spanner until progress was clearly happening.
It was all standard kit I took off, dirty but functional... Old pads/discs with uneven transfer of pad material to the front discs so it pulsed on hard braking.. Lipped discs font and back, squeaked like **** too! Slide pins all moved easily enough, perhaps a little rough in patches... A variety of different colour greases was applied to the front slide pins/bolt when I found them mind.. So someone has been in there before and gooped them up.
Pair that with decent Goodyear tyres, and the tyre-tarmac friction was always greater than the pad-disc...
Still all seems to have cleaned up nicely now.. Fresh lick of paint, silicone paste lube on all the slide pins, a conservative smear of Ceratec on the pads to help them glide nicely... Smooth and silent the whole way
I tried many way to break the nut free from the pipe! Bathed it in penetrating oil, tried tightening, loosening it, more penetrating oil, wiggling back and forth, turning the nut as far as I dared despite the line starting to twist, more penetrating oil, chocking the brake line to prevent the loop section acting like a lever on the line that disappears under the body etc.. All with the hose union held in place by a 17mm spanner.. These methods all worked at the front, but no joy at the rear...
Since I have neither a brake line flaring tool, spare brake line, unions, nor experience in any of the previous, I figured it wasn't worth pursuing just for the sake of replacing a set of perfectly functioning hoses with braided ones.
Now if anyone has a step-by-step procedure for the ABS unit cycling (8 valves, 1 motor?!?) I'd appreciate being pointed in that direction!