The water pump

On a Toyota Prius, there are actually two translucent (“plastic-looking”) coolant reservoirs under the hood:

1. Engine coolant reservoir (overflow tank)

2. Inverter/electric system coolant reservoir

So yes—what you’re seeing is very likely an overflow/expansion tank, but which one matters for diagnosis.

🔍 What that plastic tank does

The engine coolant reservoir:

• Holds excess coolant when the engine heats up

• Feeds coolant back into the radiator when it cools

• Should stay between “LOW” and “FULL” marks—not empty

If it keeps emptying, that’s not normal—it means coolant is going somewhere.

⚠️ Your symptoms point to a real issue

You said:

• Known bad water pump bearing

• Reservoir empties after refill

• Overheating warning goes away after topping off

That combination strongly suggests:

👉 Likely diagnosis:

failing water pump leaking coolant

The engine water pump on this Prius:

• Has a shaft seal that often fails when the bearing goes bad

• Will leak coolant (sometimes only while running or hot)

• May not leave a big puddle—you’ll just “lose” coolant over time

So you’re probably seeing:

• Coolant leaking out of the pump

• Reservoir draining as the system tries to compensate

• Temporary fix when you refill it

🔧 Other possible (but less likely) causes

Still worth considering:

1.

Radiator or hose leak

• Look for white/pink crust (dried coolant)

• Check under the car after parking

2.

Coolant control valve (common Prius issue)

• Affects coolant flow, but doesn’t usually cause fluid loss

3.

Head gasket issue (more serious)

• Signs would include:

  
  

  • White exhaust smoke

  • Rough startup

  • Bubbles in reservoir

• 
  

• Less likely if topping off fixes overheating quickly

🚨 Important reality check

If the reservoir is going empty:

• The system is losing coolant somewhere

• Continuing to drive risks:

  
  

  • Overheating

  • Engine damage (very expensive)

• 
  

🧠 What your exact pattern suggests

Given everything you said, the most probable chain is:

Bad water pump bearing → seal failure → coolant leak → reservoir empties → overheating → refill temporarily restores cooling

This is a very common failure pattern on this generation Prius.

✅ What to do next

• Inspect around the water pump area (passenger side of engine)

  
  

  • Look for pink residue (Toyota coolant)

• 
  

• Replace the engine water pump ASAP

• Refill and properly bleed the cooling system

💡 Quick tip

Make sure you’re checking the engine coolant reservoir, not the inverter one—the inverter system has its own pump and tank and is separate.

If you want, you can send a photo of the exact tank or engine bay—I can tell you which reservoir it is and pinpoint the pump location.