How to find correct part number for Audi?

If you don't have the original one, you're bolloxed. Best thing you can do is to measure the distance between the two studs on the servo that hold it on and the internal diameter of the large hole inbetween them and also count how many pipes are leading up to it. From there, you should be able to find one that fits the servo with the correct number of outlets and it'll work, even though it's not the original.

Nedavno Conor napisa:

I believe that all of these will fit my car (I guess that Audi would not list them for it otherwise), and which one is exactly for my car depends on the type of breaks that I have (heavy-duty brakes or not). But I do not know which type I have.

Local parts store has one for my type of car, but listed for car w/o ABS. What difference does that make? With or without ABS, master cylinder should be the same, right?

Well if it's an estate, long wheel base or turbo you need the last one. If not then you can look at the calipers to find out if you've got heavy duty brakes.

Nedavno Duncan Wood napisa:

No, I found the photo on the Internet, and the last one 25,40X17,5/18 has different brake fluid reservoir, so I guess that that leaves me with 23,81X19/17.

What does 19/17 stand for?

IIRC the first number is the bore, the 2nd&3rd are the stroke of the pistons.