Andy's metaphor once was to think about breathing as a bellow. You'll have to find where/when tension sneaks in. It might be still in your first two breaths. I'd suggest the checklist:
- Is your neck relaxed and aligned with your spine?
- Do you have the feeling your head is just floating?
- Does the relaxation hold, while you're nodding/turning your head?
- Is your leading hand/arm relaxed from deepest up fingers-wrist-elbow-shoulder?
- Does your leading hand stay relaxed and fixed on it's wide rail?
- Is your core stable with enough relaxation to breathe unhindered?
- Are your legs aligned with spine and relaxed?
- Did you try inhaling down to your stomach... and will you let this deep air out again by itself (relaxed)?
You also may try in shallow water: Take two strokes and exhale in your usua way. Hold the rest air and stand up. If you can let out some air just by relaxing without pressing, you didn't exhale enough, because there has been some tension left...
My wife likes to think breathing as 6/8: 2-outflow, 2-outlfow, 1-rest air spit off, 1-inhale...
Hope there's something to study and find something out...