2000 Opel Corsa Rusty water in water pump

You didn't say what has been "fixed" that wasn't needed so I can't comment on that, but what have you done after you get the car back and what you took it in for is still acting up? What did the mechanic say? There are a lot of problems, especially those that act up intermittently that are hard to know when they're fixed properly unless you bring the car back. When the mechanic knows the car's recent history, he can work his way from the most obvious or common cause of a problem to the less common ones, but you don't want him to tackle the more expensive or less common fixes first. He often has to resort to the process of elimination, and let you drive the car in between repair attempts. Some owners understand that and some unfairly call the mechanic incompetent. It's funny that mechanics are held to much higher standards than doctors. When doctors don't get it right the first time, we keep going back until they do, and we get charged for each visit.

I assume you're seeing the rust-colored coolant in the reservoir, not the water pump. You can't see into the water pump, and whatever is in it is the same as in the rest of the cooling system. One possible cause is a leaking cylinder head gasket. That is the less common way for one to leak but it can allow engine oil and coolant to mix. Look at the oil level. If it is higher than it should be, the coolant is likely getting forced into an oil drain back hole through a leak in the head gasket. That will turn serious in short order if it is neglected too long. Antifreeze will melt the soft first layer of the engine bearings and it will reduce the oil's ability to isolate moving parts from each other. In other words, it loses its lubricating ability. The problem must be corrected and that oil with coolant must be drained right away.

Cars with automatic transmissions can have a leaking transmission cooler inside the radiator too that will cause mixing of those two fluids. You don't have to worry about that with a manual transmission.

If the only symptom was loss of coolant, and the color wasn't an issue, a leaking head gasket would be the most common cause. Your mechanic can do a chemical test at the radiator to check for that. The test involves drawing air from the radiator while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.

For a slower loss of coolant when no wet areas can be found, you can add a small bottle of dark purple dye to the coolant, then search a few days later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. With a leaking head gasket, you'll find the dye inside the tail pipe. If coolant is leaking into the oil, you'll find the dye on the dip stick and under the fill cap.

Sorry I couldn't reply last night. When the "not responding" messages strike, my computer locks up for anywhere from a minute to an hour, so I just give up and turn it off.

The temperature gauge on the dash is never terribly accurate. To know exactly how hot the engine is, your mechanic will connect a scanner that displays live data during a test-drive. It is fairly common for the temperature to appear to go too high at first because the temperature sensor for the gauge is not next to the thermostat for the cooling system. The hot coolant gets to the sensor before it migrates over to the thermostat causing it to open. When it opens, cold coolant comes in from the radiator and the dash reading will go down a little. The point is, if the engine doesn't overheat, what you're seeing might be normal on a hot day.

For the loss of coolant, consider using the dye and black light. Auto parts stores will have the dye, and many of them borrow or rent tools and they should have the black light too.