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Okay first I will explain the operation of the throwout bearing to you. The throwout bearing reduces friction between the clutch fork and the pressure plate. It is moved into the center of the pressure plate causing the pressure plate to pull away from the clutch disc which releases the clutch disc from the flywheel. When you release the pedal, the spring pressure inside the pressure plate pushes forward on the clutch disc to lock the clutch disc, flywheel and pressure plate together. The engine can then rotate the transmission and gears, thus operating your powertrain to move the vehicle.
Now onto the problem being a bad throwout bearing. The problem could in all reality be several different things.
1. clutch slave cylinder leaking and empty, not bled properly.
2. Clutch fork not installed properly or secured properly and could come off of throwout bearing.
3. If it is a constant running release bearing then correct preload must be set during installation, could be wrong preload setting.
4. If the vehicle is equipped with a concentric slave cylinder, then there is a retaining ring that holds the release bearing to the slave cylinder and that could of fallen off.
5. Could of just gotten a bad clutch set or a bad part in the clutch set, this can happen from time to time..
Without seeing the vehicle and performing the removal and inspection of clutch set I would just be speculating as to the root cause of this. I have listed above different possiblities for this problem and please understand I am in no way placing anyone at fault here as I cannot physically see vehicle and inspect. However, If you paid them to do the repair and they supplied the parts then you should in no way be responsible for any repair fees this time if in fact it was only driven by you for 40 miles after the repair. Their warranty and the warranty of the manufacturer of the parts should cover this.
I hope this answers your question, and please remember if I was helpful the please
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Good Luck and Best Wishes