K-H brake swap done, but doesnt stop!

It’s not common for it to in my experience. I suggested that as a starting point to see where fluid is making it to, but it would be easier to crack the line at rear hose from the steel line off the body. If nothing starts dripping out when you take it loose, you should be able to put a piece of tubing over the line and put the open end into a container and get some fluid out of it by pumping the brakes. If it’s the rear hose not letting fluid pass, you should have fluid almost as soon as it’s cracked open. If it does, retighten it and then crack the lines loose at the Y-Block on the axle to see if fluid starts to come out. If the hose has to be changed, the Y block has to be removed and then screwed onto the new hose.