hi there do u mean the brake master cylinder?
if so yes there are differences lol i found out as had mine replaced as hunting this wierd prob i have that no one can fix.
anyway depending on your tdi it may have the esp which i was told makes the difference if you searching ebay for one then you will need the part number which is not situated on the master cylinder but on the servo which the master cylinder is attached to and sits behind it on tomards the driver it looks like big black spinning top(lol depending on age u might not know what that is) or dome. i ended up getting one new through mechanic for i think was 135 ish.
there suppose to be bullet proof and after my new one it never made anydifference to my pron so all in all i just payed money to make no difference.
what the problem u have?