We have a 1 1/2 sink unit which was installed approx 2 years ago. I've noticed the middle unit has been leaking slightly into the cupboard below. On closer examination it looks like the end cap is leaking through the rubber seal on the end which looks like it may be perishing. What I can't understand though is why this cap has a rubber seal - why can't it it just be a solid end cap? Thanks in advance.