My line is 5 ft long and 1/4" i.d. x 3/8" o.d. is marked on the side. I bought it at a homebrew store. The fridge is set at around 35 degrees. After the initial day at 35 psi, I've had it constantly on at 12 for about three weeks.
I was sure to check for leaks at the beginning and I purged the O2.
In my opinion 5ft is too short especially for 1/4" id hose. You need something more like 8ft. 5ft will get you a rough and foamy pour at 12psi... that rough pour will leave your beer flat. That is probably your problem.
Additionally, leaving your beer at 35psi for a day may result in overcarbed beer. It's hard to say (and that's the problem with the high pressure force carb method - I prefer the "serving pressure+shaking" method). The fact that you later set it at 12psi and leave it there does nothing to solve an overcarb problem if you have one.
The problem is there are 2 reasons for foamy beer and they are not mutually exclusive.
1. line length is too short giving your serving pressure and line diameter.
2. beer is overcarbed.
You may have both going on but it's impossible to say because the overcarbed beer (if it is overcarbed) will be rendered flat by a rough pour which you no doubt have given your short line length.
I say start by getting a longer length of line and see if that resolves it. Start with 8ft minimum, maybe 9. You can always cut some off but you can't put it on. If the pour is too gentle then cut some off. But I'll bet 8ft is the sweet spot.
If the beer is still foamy after you get the line length right then you have it over carbed and you need to rethink your force carb method. Again, in my experience the high pressure method is imprecise and a total gamble.
If you must force carb then do the following:
1. get the beer at serving temperature
2. attach gas line. It works best if you attach it to the beer diptube.. i.e. put a black beer disconnect on your hose line and snap it to the 'out' post on the keg. This way the gas bubbles up through the beer as you shake.. it expedites the process. You don't have to do it this way though, but it helps. Do be careful though to not set pressure in the regulator below pressure in the keg or you will backflow beer into the regulator which will be bad!
3. set regulator at desired serving pressure (e.g. 12 psi)
4. shake the heck out of the keg until the gas stops flowing into it.
5. let beer sit at least overnight before serving to allow the foam to settle.
6. attach gas to gas post and attach beer line to beer post.
7. drink up.
