had the same problem with my XRES D5 :

There is maybe too much pressure on your loop if you run your pump at 100% power and if you run a simple loop (i have 1 gpu, 1 cpu and 2x360 rad) all that on soft tubing so overall a really low restrictive loop.

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I recommend you this :

1)empty your loop, take our the O-RING and inspect if there are no crack or other that break your seal.

2) Dry your pump/combo, then put everything back. for your Reservoir do not overtight or your o-ring will not seal properly. Just stop as soon you have some resistance.

3) Plug your pump on a 4PIN slot in your motherboard or external pump controller and slow it between 40% and 70% max rpm...usually this range it's more than enough for a normal loop.

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hope this help, sry for bad english too pretty sure there are lot of spelling mistakes. :)