I've searched this site for this issue but it appears other questions/answers relate to old cables/housing. This specific problem happened after replacing both (link to Shimano brake cable set- https://www.amazon.com/gp/product/B0050LUBZ8/ref=oh_aui_detailpage_o03_s02?ie=UTF8&psc=1).
Having neglected my brake cables/housing for many years, I've gone ahead and replaced the cables and housing on my 2012 Trek 8.3 DS. This seemed like a very straightforward repair, so I went for it. Interestingly, the old cables didn't have much rust on them but I went ahead and replaced them anyway assuming the stretch would affect performance.
Long story short, I cut everything to essentially the same length and fed the cables/housing through all the various caps and grooves in the exact same way the old, factory installed cables were. Upon squeezing the brake lever I am finding some of the housing shifts and even pops out of the grooves on the underside of the top tube. This, of course, "resets" the tension and suddenly I have a significant amount of slack in the cable.
I should mention the end caps on the housing were a wider diameter than the pre-existing ones so they didn't fit well into the plastic caps that preceded bare wire running the length of the top tube. Reading the product description again, I'm seeing the metal pieces are all considered end caps (MTB/Road). I put these onto the housing where the housing ends so perhaps this is causing problems (the original housing has metal caps at the ends).
After reading up on this site on similar issues regarding older cables/housing, I see many asking if the cables had recently been lubed. I've read in a few places I shouldn't be lubing brand new cables since this will attract dirt. I'm not sure what to do since this friction within the housing could also be shifting the housing around after applying pressure.
I'm also noticing there is zero "spring" tension pulling the cable back after applying pressure to the brake lever. Might this also be a factor?
Thank you for your help!