Washing m/c & Dishwasher leaking on shared water inlet

[ST8]

Whilst fitting a new dishwasher the Y joint cracked. This has been replaced by a new one from the local merchants but the first attempt resulted in a face full of water .So I returned to the merchant for advice and fitted an extra washer. This seemed to have solved the issue until I was woken by my down stairs neighbour informing me of a leak. The only further advice available from the merchant was to replace the Y joint as the thread had probably be crossed. This was not the case but the new new one still leaks.
Any advice would be greatly appreciated as i fear a divorce is imminent.

 

[fido]

As long as there are 3 rubber washers, one for each joint, you should not have a problem. I take it the Y piece has a nut that will turn independently of the rest of the fitting? There are some where this nut is moulded on so you have to turn the whole fitting to screw it onto the tap. If the tap is in a confined space and the joints are being strained you may get leakage. Also make sure both your hoses are OK. You could check these by temporarily connecting them one at a time without the Y piece fitted.