There are still to types being sold. A brass taper fit unit with an o ring seal, and the straight fit nickel coated brass type with a neoprene [soft plastic?] insert.
The brass ones can get very stiff after some time, to the point where you will bend the tap handle trying to turning. I have tried several things including lubricating and smoothing with 800 grit emery, none worked 100%.
The neoprene ones work great for years but the neoprene can swell with fuel and slowly closes off the rate of fuel getting through. To fix this you need to take the tap off and hand twist a new drill bit through the tap while it is open. this cuts the swollen part of the neoprene away and opens the tap again. You may need to do this several times over the life of the tap [maybe not, maybe yes, its hard to know, just gotta suck it and see], eventually the amount of neoprene removed affects the seal of the tap and you need to replace them.