The vented loop should contain a small valve at the very top of the vented loop hardware. This valve allows air to flow BACK into the loop line thus breaking the suction caused by any 'siphoning' back into the system when the pumping is finished. If the valve sticks open .... shit-water comes out and when the 'fluid' finally equalizes its 'level' in the loop allows the 'gases' to escape through the stuck-open teeny relief valve.

Whats happened is that this small (usually molded rubber) valve has either destroyed itself, become totally encrusted with 'calcium' deposits, .... OR the discharge line/hose connected between the 'toilet' and the TOP of the 'loop hardware' has become so encrusted with calcium deposits that the deposits are now so thick that they are interfering with action of the 'relief' valve. Its not uncommon after years of service especially in seawater to have this 'connection' line to be COMPLETELY blocked by thick calcium incrustations.

Open the small 'boss' on top of the loop hardware AT the 'air bleed', extract the valve and either replace it or clean it - siimple job.