hi i had a bathtub that drained fine for about a couple months in a older house new to us . recently it started draining very slow and you would only be in the shower for less than a minute to have standing water .and the kitchen sink would burp . i see on the top floor landing is a vent that goes to the roof however in the basement i think someone replaced the tub or bought the wrong one because the vent is on the one side of the wall and all the plumbing to the tub is in plain veiw and there is no vent tube hooked up . the kitchen and bathroom sink are connected into the tub drain pipe and ran out of the wall . and i dont know where from there we dug up septic several times from leaching problems only to find out that the tub and sink drains dont go into septic tank they just go out back somewhere into a drywell im guessing . anyways i am going to be doing this all over in the spring and i am just wondering if anyone has any ideas. i have a very small tank so i have to do a dry well of some sort but do i need a vent for each of the sinks and the tub and combine all of them into one ? and if so do i place them just on the outer side of the drain traps . any help is greatly appriciated thanks ..