* Heat-related expansion and leakage: Heat causes the power steering fluid to expand. If there's a small, slow leak in a hose, seal, or the power steering pump itself, the increased pressure from expansion during summer heat might be enough to push fluid out where a smaller leak wouldn't be noticeable in the colder, lower-pressure conditions of winter. The leak might be in a component that warps slightly under heat.
* Fluid degradation: High temperatures can degrade the power steering fluid, making it thinner and more likely to leak past seals that are already slightly worn. This is particularly true if the fluid is old and hasn't been changed.
* Hose deterioration: Rubber hoses can become brittle and crack with age and exposure to sunlight and high temperatures. Cracks might only open up enough to leak when the fluid expands in the summer heat.
* Increased pressure: Higher ambient temperatures can sometimes cause slightly higher operating temperatures for the power steering system, potentially increasing pressure and revealing a marginal leak.
In short, the summer heat is acting as a stress multiplier, revealing a pre-existing weakness in the power steering system that isn't noticeable in winter due to lower temperatures and pressures. You need to have a mechanic inspect the entire power steering system—hoses, pump, rack and pinion, pressure lines—to pinpoint the leak.
