I use Perlick 545s on my kegerator and I am a huge fan of them functionally.  It is great to have the ability to serve 2 volume to 3.5 volume beer and not have to change anything in my kegerator except rotate the flow control on the 545.  They also provide a nice smooth pour and the forward closing setup does work to prevent sticking.  The issue is they use a pretty complex design which, in general, usually leads to reliability issues and these are no different.  The have a tendency to leak if anything goes wrong – a little bit of hop matter gets in there, the pressure is a bit high, if the wind blows from the west etc…  A good cleaning and swapping o-rings if any are damaged usually does the trick.   That said, there are 8 billion o-rings so I finally decided to figure out what size o-rings are used so I can buy them in bulk rather than way overpaying.  I do the same thing for keg rebuilds and have saved a bunch of money over the years.  Also, Perlick seems to only sell a subset of the o-rings – the ones in the bonnet, the one in the front that actually controls beer flow and the coupling gasket.  the 3 o-rings that are part of the flow control assembly don’t appear anywhere on their website from what I can tell.So, I measured all the o-rings with digital calipers and have ordered 100 packs of all of them so I’m good for a while (forever?).  Here is my partlist from McMaster Carr:
B1&2
9452K58
Buna-N O-Ring, AS568A Dash Number 014, packs of 100
C
9452K338
Buna-N O-Ring, AS568A Dash Number 204, packs of 100
D
non-standard
L
9452K21
Buna-N O-Ring, AS568A Dash Number 012, packs of 100
N
9452K59
Buna-N O-Ring, AS568A Dash Number 015, packs of 100
O
9262K232
Metric Buna-N O-Ring, 2.5 mm Width, 10 mm ID, packs of 100
And here is where they all go: