Several possibilities
No, you can't just leave it alone. The air valve in a vented loop should only allow air into the line, nothing should escape. They do require periodic cleaning and occasional replacement, so that's the first thing to do. The backpressure is another clue...possible causes: The most likely is a blocked holding tank vent. When air displaced by incoming waste can't escape out the vent, the tank becomes more and more pressurized with every flush. When that happens, air tries to escape anywhere it can, including the air valve in a vented loop. 90% of vent blockages occur in the thru-hull...so that's the first place to check. However, the easiest way to find out if that's the problem is by VERY CAREFULLY opening the cap on the deck pumpout fitting. Have a hose at the ready, 'cuz if the system is pressurized, air and/or waste will erupt. Do NOT try to pump out the tank until you're certain the vent is clear...pumping out against a blocked vent only pulls a vacuum that will prevent the tank from being pumped out and can result in a cracked tank. The next possibility is sea water minerals and salt buildup in the head discharge hose (I've seen it reduce a 1.5" hose to less than half an inch). When the hose is restricted, there will be backpressure when the toilet is flushed...and when there's backpressure, air has to get out somewhere. The air valve in the loop is the closest place. Remove the hose from the head discharge fitting to see what it looks like inside. Finally, there could be a clog in the hose. However, if there is your toilet will have a very hard time flushing at all...the bowl won't want to empty. So my money is on either the air valve in the loop or a blocked tank vent. But whatever the cause, you need to find and correct the problem...don't even use the toilet again untill you do.
