Most of us know about the hydrostatic gel that helps absorb impact in MJOLNIR armor. It is also mentioned many times in the novels (particularly First Strike) that it is vented in emergency situations, such as intense heat. On the surface, this seems like the best system.
But, that gel is necessary for absorbing the force of falls, collisions, etc. Wouldn't there be less (hence, less protection) after, say, being blasted with an Assault Cannon?
Also, some of it might leak onto the bare flesh of the Spartan. Here is my idea of what that would look like.