My pump box is also the same as this (this may even be a copy of the photo I posted ages ago??? Not sure)...
I have found recently that I was getting a loss in pressure, even with the pump on FULL I was only getting a 20% output in water!!
Solution - I changed ALL the pipes in the box for new pipe. Apparently, after a while, something in the pipe and/or brade, reacts with the water and eventaully the seal can go between the pipe and the T connector (or any of the other connectors for that matter), even though there isn't any movement in the pipes.
Cleantech sell the clear braded pipe from stock at a reasonable price, and you can get the thin black pipe that some use on their actuall WFP poles/brushes, from B&Q for £3 ish.
Cheers
Steve