Kitchen drain backup with dishwasher

Kitchen sink backs up into other sink when dishwasher or disposer run and drains slowly. Trouble started when we added dishwasher and disposer. I have 10 inches of drop from sink to trap is this enought? Replace inline vent about drain trap, checked 1.5 inch line it,s clean and has 1/4 inch drop to 4 inch line. Have 20 feet of 1.5 inch line to 4 inch line. Is this line too long? Does 1.5 inch line have enough volume to handle dishwasher and disposer? Should I go to 2 inch line to handle the volume?

Thanks
egor

My money is on your "inline vent". Technically they should not be used in most juresdictions. The vent should really connect to a natural vent that goes up to and through the roof. You can test that vent/drain by simply removing the avv and try using your dishwaser and or disposer. If it runs good, then that is the problem.

Added hight to vent and check complete system. All system go!
Thanks
egor: