Not having the correct sized master cylinder will increase the likelyhood of getting "ice mode" with the brakes. This is most eveident on the trake for those who have upgraded to 6 piston brakes from 4 piston brakes. Swapping to the PCCB master cylinder as Der Boost has talked about reduces the frequency of "ice mode". Much discussion about this on Rennlist.

 

Just for all the 997 newbies like myself who might be looking at this thread and thinking "Should I be upgrading to the 27mm too?" Well the answer is no, they already come with it.

 

I made a custom line for mine in order to do it the right way. Ever since I have upgraded to the 997 master cylinder I get ABS/PSM codes every now and then. If I am not mistaken, many others have had this problem as well from past threads. I had intentions of upgrading my brakes but decided not to do it since I will not be tracking my car anymore. So, I will eventually reinstall the original master cylinder whenever I get around to it - the stock unit is fine for the street and most applications.

 

I've been running the 997GT3 master in conjunction with 997GT3 380/350 brakes for the last two years. No issues, no warning lights. I do not have PSM however..