Engine/Transmission (1994 - 1998) Nut specs for master cylinder bracket

[Jfam]

Once upon a time (2006) I had a big problem with my 1994 12 valve. I had a Prime Loc fuel filter re-locator on at the time and most everyone on the TDR told me it was the problem. It was not. Piers diagnosed the problem correctly as the IP. I wish I had left the Prime Loc on as it is SOOOO much easier to change the fuel filter and I also had a low pressure fuel sender attached. Anyway, I go to change the filter today and following the good advice of TDR members over the years start to remove the two bracket nuts for the master cylinder. I remember having trouble last time I changed the filter, but could not remember what it was. The left lock nut is a smaller diameter than the right (11/16) It APPEARS to be 9/16, but that would not work. Sure, the nut had been mangled over the years. What I ended up using was a 19/32 socket. OK no problem. When reassembling the unit I TRIED to use a new 3/8 lock nut with a 9/16 head. Though both the old and new nut fit perfectly on a 3/8 bolt, only the old would reassemble to the master cylinder stud. What the frig? Anyone know what is going on? Thanks.....