Need help finding drainage issue.

I have a bathroom sink which drains slowly. When we run the washing machine water comes up into the bathroom sink so we'll just pause the washing machine during rinse cycles to allow the water to drain some instead of overflowing the sink. I have run a snake the full 25ft both down the bathroom sink and the drain pipe for the washing machine with no blockages discovered nor did this change the problem. I then took the little rubber piece off of the vent under the bathroom sink and ran water at full blast from the sink and it drained the water just as fast as it was coming out of the tap. We then ran the washing machine but during the rinse cycle water came out of the vent (which still had the rubber piece removed). I have no idea what to check/test next. Any suggestions are welcome.

Thanks,
Alex

Sounds like vent related. Do you have access to the entire vent stack up through the roof?

I do have access. I've already ran a snake down the roof vent which made it to the ground. It was a very old snake that wouldn't rotate and it seemed to make it to a part that does a 90 degree turn and I couldn't get it to turn. The roof vent goes straight down and the washing machine drain connects to it before it makes it to the ground and turns in the crawl space. I live in a double wide where all the plumbing is done on one side and drains into a septic tank if any of this helps.

You may want to try using a 4 inch disk plunger that you can buy at home depot for less than $10. This assumes that your vent stack is 4 inch). It is a flat rubber disk with a brass center. You thread on a length of black pipe about 8 feet long (length depends on the length of your vent), and you essentially have an 8 foot long plunger. Push that up and down a few times in your vent stack.

My roof vent pipe is 2" diameter. I've also already ran a snake down it while I had water pouring in from a water hose. Snake came back with nothing and the water never backed up anywhere. I just don't understand why the bathroom sink will overflow from the washer when the tub and shower (which are in the same bathroom as the sink) have lower drains but don't back up at all.

Thanks for the replies so far!

have you tried pulling the trap plug on the washer drain and snaking from there????