If you have difficulties in determining controller parameters, you may want to omit the D action as a first step. I action is generally required to remove a static error for most control processes. If you set I gain rather low, it shouldn't cause problems but still achieves zero error in steady state.
Ziegler-Nichols tuning rules provide a rather easy way to perform an acceptable tuning for usual control processes without complex calculations or signal processing. A pressure controller possibly involves issues as non-linear actor characteristic, simple tuning rules may be inappropriate in this case.