Here's every reason that I built my own loop.
This is my cpu blockhttp://www.ebay.co.uk/itm/100-New-CPU-Water-Block-Water-Cooling-Cooler-Computer-Radiator-for-intel-AMD-/331817277836?hash=item4d41d9d98c:g:0h8AAOSwdU1W-1-N
basically identical to this one. I thought £15 was cheap. It works just great. I have resolved my pressure drop problems and it wasn't to blame so I have no problem recommending it as a suggestion.
I would buy this one again + consider buying the same style of gpu block & northbridge cooler from the same seller in hong kong as well. & I lapped mine, disassembled it and plated it in silver with antiquax silver plating soln.
You would need your own pump radiator reservoir tubing and couplers for whatver design enters your mind. The point being that if you buy each part and join them with standard plumbing couplers or hose clamps then you can fix each one individually.
Here you're stuck with stuff that you don't know what the (poisonous) coolant may be; whether the rad is copper or aluminium; what's causing the pressure drop/is there air in the radiator and as it's closed loop you can't top it up without invalidating the warranty
So what are your choices? Well you could just snip it open drain and refill it test each component & the flow through it (what a lot of hassle), take it back to the shop or re-use some of the parts in your own custom loop.
You can easily join tubes if you sleeve the smaller in the larger; eg. stuff 12mm Outer Diameter into 12mm Inner diameter(which would likely be 15mm OD tubing) as far as it will go and then secure it with a worm screw hose clamp. I have found this to be a waterproof tube coupling method that works under a load of at least 37c; my water loop has not exceed that temp
So as long as you know how to join hoses and what couplers to use to join the hoses to the rad. and block you can do what you like.
Why do closed loop WC's exist really? They are like the celeron or the stock heatsink; what's the point of it? Whatver happens you will a)never buy a celeron b)never use the stock heatsink and c)always snip open the closed loop WC therefore you might just as well make your own open loop.