I just found this... its in Spanish but the wonder of the internet can translate...
https://www.revistamash.com/2017/detalle.php?id=261
View attachment 32672
The yellow part of the images is 'overcarbonation' - which grows over time and ends up taking over the remainder of the keg. Sounds like thats whats happening to you. So maybe this is suggesting you should vent your keg and drop to around 2psi for serving? Not sure if this helps but thought i would share what i found.
Here's the English from google:
Normally, a full barrel begins to be discharged that has its ideal pressure (around 1bar). To push the beer, CO2 is injected with some pressure so that the pressure on the manometer remains around 1.5 bar. The surface of the beer inside the keg receives this pressure, but the liquid is only 1 bar. According to the Gas Diffusion Law, the higher pressure will penetrate the liquid mass. At first, the beer will continue to come out normally, since the supercarbonated layer is very thin and the liquid is taken from the bottom of the barrel. But with the passing of the hours, the greater pressure diffuses inside the container and reaches the bottom, beginning to come out overcarbonated beer.
What is the solution to this problem? Some years ago, Guinness technicians observed that traditional British beers that were naturally aerated in barrels and were propelled by air from the bartender's "engine" were oxidized by the inlet of oxygen, but did not alter their gassing. After studying the issue, they came to the conclusion that this was due to the presence of nitrogen, the majority component of air (78%), which is poorly soluble (80 times less than CO2). They began to do tests with mixtures of CO2 and N2, until they found the ideal mix for Guinness (75% CO2 and 25% N2). This mixture, known as G Mix, is ideal for the level of carbonation (1.2 volumes) of Guinness, but it is not satisfactory for other beers that require different volumes of carbonation. On the market, mixtures of CO2 and N2 are offered with variable ratios, which can be defined by the user. The McDantim company from the USA (from whom we have taken part of the material in this note), offers on its website, an Excel spreadsheet that allows calculating the CO2 and N2 ratio for a certain number of volumes at a certain temperature at normal pressure at sea level. Likewise, as it may be required to serve different types of beer with a single serving equipment, they have created an “in situ” mixer that allows working with separate CO2 and N2 tubes, which ensures the quality of the beer served and greater profitability for saving beer. which can be defined by the user. The McDantim company from the USA (from whom we have taken part of the material in this note), offers on its website, an Excel spreadsheet that allows calculating the CO2 and N2 ratio for a certain number of volumes at a certain temperature at normal pressure at sea level. Likewise, as it may be required to serve different types of beer with a single serving equipment, they have created an “in situ” mixer that allows working with separate CO2 and N2 tubes, which ensures the quality of the beer served and greater profitability for saving beer. which can be defined by the user. The McDantim company from the USA (from whom we have taken part of the material in this note), offers on its website, an Excel spreadsheet that allows calculating the CO2 and N2 ratio for a certain number of volumes at a certain temperature at normal pressure at sea level. Likewise, as it may be required to serve different types of beer with a single serving equipment, they have created an “in situ” mixer that allows working with separate CO2 and N2 tubes, which ensures the quality of the beer served and greater profitability for saving beer. an Excel spreadsheet that allows calculating the CO2 and N2 ratio for a certain number of volumes at a certain temperature at normal pressure at sea level. Likewise, as it may be required to serve different types of beer with a single serving equipment, they have created an “in situ” mixer that allows working with separate CO2 and N2 tubes, which ensures the quality of the beer served and greater profitability for saving beer. an Excel spreadsheet that allows calculating the CO2 and N2 ratio for a certain number of volumes at a certain temperature at normal pressure at sea level. Likewise, as it may be required to serve different types of beer with a single serving equipment, they have created an “in situ” mixer that allows working with separate CO2 and N2 tubes, which ensures the quality of the beer served and greater profitability for saving beer.
Note this seems to be about solving the issue that you need more pressure to dispense than you do for carbonation which is unlikely in a home brew setup.
