I have the same problem, and it's always done it, which is annoying.

I think the valve is faulty, but it's hard to find the original ones and unless new, it's likely to have the same issue.

A few years ago, I got an aftermarket one, but this was worse than the original, as it run too fast, I figured the springs were not of the correct specification, but even messing around with them, it still would not work right and so I put the original back on.

Martin