I heated up the booster end of the original solid pipe so the part that pushes in to the booster stays there and you can then fit the red pipe I put on , its from ASH hoses , of the top of my head it was like 8mm or 10mm ID I think but ill have to measure it again to be sure sorry , BUT , ive found it does close up under heavy breaking , no problems from inside the car but you can see it squashed up in the bay so I would recommend either short silicone hose on to a solid plastic tube , OR buy a cupra R brake booster pipe on ebay for like £30 .