1/4" line has a pretty low restriction - the numbers vary by source, but I've seen 0.65-0.85 psi. The line may still be too short for 8 psi.
Beyond the actual pressure on the keg, how many volumes of CO2 are in the beer and what is the temperature? If you have a check valve in the keg and if the CO2 is coming out of solution in the keg, then the pressure in the keg may be higher than is registering at the gauge. This could also cause the problem.