I have just fitted my roof and sides and had an old lock barrel from when I replaced the fronts. I figured it would fit but it makes no difference to the lock, it just pulls out.
Are the rear barrels different/longer than front ones?
Sounds to me like a leak on your brake pads and disc, keep pumping and see if you reservoir drops further.
I would not drive it anymore until you're sure!!!
The trouble is "the banter" we are all used to as longer term members can come across as rude to newbies and unfortunately that "can" scare of/**** of potential new members with an awful lot of knowledge and info.
I had a home made cover on my pick up but swapped it plus some cash for roof sides etc. In the process of cleaning them up for fitting.
Cover is easy to do. One sheet of chequer plate 1.5inches bigger on all sides, bent to fit then add some struts. I hinged it under the rim from a canvas hood.