I've sequentially upgraded my spindle cooling from a pump in an open bucket (very bad) to sealed water+glycol reservoir, with a CPU-cooler heat exchanger, and a pinwheel flow indicator. On a replacement 2nd spindle now since water channels in the first spindle plugged up (that pump in an open bucket collecting dust problem!). Still the flow would decrease over time for unknown reasons. I would swap tubes at the pump/return to reverse flow and notice some improvement.
Today I installed a reversing valve in series with the two lines to the spindle. An inexpensive but serious-looking thing from Ali Express. Intended for pneumatics, but hey, something rated for 100PSI air should be fine for water+glycol at fractional PSI. Improvement was dramatic. After a couple toggles of the valve, the pinwheel is spinning like crazy, back to how it was when I installed the new spindle.
I think in spite of the sealed system, very small bits of cruft collect and stack up upon each other in the contorted spindle cooling channel, possibly landing upon machining flash and whatnot. Slamming that reversing valve back and forth really seems to break something up and clear the channel. Very pleased.