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It is highly unlikely that all 4 would have the same type of problem at the same time. Where is the valve leaking? If corrosion is an issue where the attach to the wheel, they should be able to remove them and clean the corrosion. If the valve core is leaking slowly, the core can be replaced or tightened.
Did you see where the leak was? If it is corrosion, I can't say that they caused that, but again it is odd. I attached two pictures. The first one is of the sensor. When the sensor is installed, it is pushed through from the inside of the wheel. The highlighted part is a seal which is pulled tight against the wheel by a nut on the outside. If corrosion is allowing that to leak, I would think they could remove the sensor, clean the corrosion, and that should fix it.
The second pic shows the valve core. The small pin like component I highlighted can leak too. That core can be removed with a special tool. It could be loose or bad. If loose, it can be tightened. If it is bad, it's a 10 cent part that can be replaced.
If you still have the vehicle at home, try this. Take a spray bottle and put a small amount of dish soap in it and fill it with water. Then, spray the valve on each wheel. If there is a leak, it will cause the water/soap to foam or make bubbles. That way you will know where it is leaking. Do that and let me know.
If the sensors are still working, they should be able to repair them, but not being there myself, maybe I'm missing something.
I hope this helps. Let me know what you find or if you have other questions.
Images (Click to enlarge)
Saturday, August 24th, 2019 AT 10:18 PM