Can anyone rell me what this is for? This hose runs from the head, up and back down through two taps. .the second tap to send to tank or to ocean. The button I'm pointing to appears to be some sort of vent but when I flush fluid squirts out of it.
Hi the thing that you are pointing to is the vented loop, the "button" is the vent designed to remain closed when passing liquid but allowing air in to the line when not, preventing siphoning.
Undo the cap "button" & remove the rubber thingy & give it a clean & replace.
If that doesn't work you can buy a new rubber valve from most boat shops.
An update on this ....
A wonderful member of the forum sent me a new button with a vent set up thingy.
Thanks Chez, it is working well. 
