They have no authority to keep your truck. They can warn you in writing and ask you to sign it, but they cannot keep the truck.

Safety issues. Bleh. That is the number one way for shops to sell work. They scare the customer into making them pay, due to safety issues.

Just go down and tell them to give you your truck. If they say no, use their phone to call the police. They will give you your truck.

You bought an old truck. It needs a lot of work. That is just the way old trucks work. Is this your only mode of transportation? What do you plan on towing? How does the truck drive? Does it pull when braking? What is the history of your truck, miles, maint history, prev owner, etc.

Don't be afraid of brake work. It is some of the simplest work to do. Yes, brakes are a safety issue, but they are not that complicated.

What city do you live in?

Those prices are rediculous. Even for the SF Bay area.