Drain pipe from black and grey

[Bob Lawrence]

Hi group,
I have had a leak for a while now in my drain pipes.
Let me see if I can explain it right.
My black and grey tanks go into a 4" abs pipe.
Somebody in the past put a pipe in the 4" pipe and glued it in.
Coming out of that they put a rubber collar and covers it and also goes
over the end of the Drain Master dump valve.
Sucker has leaked for a couple of months now at the glued in 3/1/2"
pipe fitting with the rubber collar.
I can't find pipe that is 3 1/2" id to go into the piece of pipe that
was glued into the 4" standard pipe of the bird.
My question is...do other people have a rubber collar going to their
dump valve?
Was this some one off job for some unknown reason?
Drain drip has to go.
Any ideas?

Bob Lawrence
84 PT36
Los Fresnos, Texas