Rear Brake Cylinder Bits... Advice needed!

We're moving slowly now, trying to get the brakes all together...

Here's a diagram of the Morris Minor brake rear slave cylinder:






Notice how there is a piston and dust cover in one piece on the right.

Here's a picture of that piston and dust cover piece from one of the slave cylinders of our car:



As you can see in this image - the dust cover cap has come off of the piston.. Looks like it was originally tac-welded.

Any suggestions for how we can get them re-bonded???