Bad studor vent or bad venting altogether?

Hi all:
We added a bedroom and bathroom onto the house three years ago. The existing waste plumbing is PVC, properly sloped, etc. All of the vents for the existing bathrooms connected to a single vent pipe with a studor vent on top in the attic. The kitchen sink also uses a studor vent under the kitchen cabinet. The washing machine drain (last one in the line before exiting the house) is vented through the roof. When the addition was done, the new bathroom vent was tied into the vents in the attic, nearby. The waste pipe from the new bathroom was tied into the existing waste piping under the house such that it flows past the point where the master bath toilet is connected. For a while, things were fine, but I have noticed recently that sometimes, when the new bathroom toilet is flushed, we get a few bubbles of air forced through the trap in the master bath toilet. This happens infrequently, but seems to be happening more often with the passage of time.
It seems as though flushing the new toilet pushes a charge of air down the main waste pipe and pressurizes it enough to force a small amount past the master bath toilet trap. However, it doesn't happen every time, or even very often. No other traps are effected. Also, it didn't always do this. Is the studor vent the problem? Does the new bathroom need to be vented separately? I'm not sure where to start.
FWIW, I don't have a problem with getting rid of the main studor vent, as the new roof penetration would not be any more visible than the existing one over the laundry room. But, because of the height and slope of my roof, I would have to pay someone to do the work, so there is some financial incentive to avoid removing the studor vent.
Any insight would be appreciated.
Regards, John.
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the_tool_man wrote:

Yes, the slug of water when flushing will create a pressure in the drain. How much pressure will be influenced by the amount of back pressure or resistance in the house drain/sewer. If the house drain is beginning to get blocked (tree roots, etc.), that may produce more pressure.
The vent for the washing machine drain *should* relieve that pressure, but there may be more we don't know.
The Studor vents won't relieve any back pressure at all; they are specifically designed to close on pressure.
Adding a vent to the new bath may not solve this since the pressure is developed further down the line, ahead of the slug of water.
It may take someone on site to look over the layout to be able to suggest the best fix.
Jim
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