The T-piece is also a non-return valve which has a small spring loaded
rubber diaphragm in it, water escaping from that little hole indicates the
diaphragm is perished and the t-piece needs to be replaced, beware, if you
replace it with a standard t-piece you will also need to install a one way
valve upstream of the t-piece, otherwise you will get a very delayed
response from the squirters every time you use them.

TTFN
Ken Phillips