brake servo vacuum pipe

I had a bit of trouble with the brakes on my disco 200tdi in the summer, only happened when parking so low speed and low revs, the brakes worked but needed more effort on the peddle, like trhe servo wasn't working, when i had a look at the vacuum pipe i found a tee piece with a smaller pipe coming off it, there was a small split right at the joint so i just snipped a bit shorter an reconnected it, problem solved, but where does this smaller pipe go and what is it for, it disappears through the bulkhead/ inner wheel arch area on the passenger side

The small pipe go's through the bulkhead and behind the dash to the heaterbox,its for the recurclating heater function which is vacuum controled

Thanks for that