I had a "dip tube" problem with my hot water heater so I had it replaced. Well, now I have a hot water heater with a good dip tube, but I still don't have hot water pressure in my bathroom, and at a couple sinks as well.
How can I flush out my hot water pipes, to remove all the decay (pvc pipe granules)?
Thank you for any help you can provide.

Do you get good hot water pressure at any fixtures?

Might be sediment which was freed up during the replacement.

Remove the aerators and clean them out,replacing them the exact way they were removed.

If this isn't the solution,you might have to remove the handles and look at the washers/seats.

Do you have hard water?

Build-up can do this over time if you do.

Kbc is correct about the aerators, unless you have cold water pressure but not hot. A plugged arerator will cause faucet to have low pressure on both hot and cold. Is this indeed what you are experiencing. If so, follow kbc's advice. If just the hot pressure in certain sinks. Then kbc also makes a good point about dissasembling faucet beyond handle and checking for debree. Please let us know what you find.