Hydraulic fluid. It is hydraulic fluid. The Numatx Squeezer is supposed to be my favorite riveting tool. It sets rivets without the noise of a rivet gun/bucking bar. It squeezes rivets the same. Every time. It is supposed to set -3 and -4 rivets but I have had poor luck getting it to squeeze the -4 rivets completely. The assumption is that there must be an air bubble somewhere in the hydraulic lines but I have bled the lines on multiple occasions without total success.
But the -3 rivets look GREAT!