Pictures are coming after I find new batteries for my mouse.

My basement has two vertical drain pipes. Drain pipe number one comes down directly from my bathroom sink. Near the basement ceiling, a horizontal drain pipe ties into it from my 2 kitchen sink drains. Both of those pipes are 2" in diameter. Near the floor, the drain from my utility sink ties into the same drain pipe. That pipe is 1 1/4". The kitchen sink horizontal run is about 6', the utility sink horizontal run is less than a foot but the grade is insufficient among other issues.

Drain pipe number two is the soil stack. The drain from the bathtub is 2" in diameter and runs just under 6' before tying into the 4" drain from the toilet, which in turn ties in to the 4" vertical drain.

I presume, but have not confirmed, that the sink drain pipe vents by tying into the soil stack in the bathroom wall, as there is only one vent pipe in the attic.

I'm looking to replace the bathtub drum trap with a p-trap, but I have concerns about the venting of this set up. As I understand it, the kitchen sink amd utility sink are wet vented through the bathroom sink. But is it adequate that the tub drain connects into the toilet drain before it reaches the soil stack? Or does the tub drain need to be vented before it ties into the toilet drain?

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