I have a 1" PVC supply line with a defective pvc ball valve. I want to replace with a brass gate valve, but the pvc line does not flex enough to allow me to fit the valve on the line.

You have to shut off the main before this defective valve, can you do that or is this your main?

I would stick with a ball valve. More reliable than a gate valve. If you want to use brass, OK. Get brass ball valve with threaded fitting, and two PVC male threaded adapters and pipe.
Cut out existing valve. Screw adapters into brass valve. Glue valve to one pipe. Cut a piece of make-up pipe to fit between valve and where you cut original pipe. Glue your piece of make-up pipe to adapter.

Make your final connection

1. With a Sharkbite slip coupling.

OR

2. Get a PVC repair coupling and some Hercules slow setting glue (tan label).
Mark the pipe where the coupling should fit.
Prime the pipe and coupling.
Working real fast, put glue on both pieces of pipe (lots of glue)
Glue inside of coupling.
Glue pipe again.
Slide coupling onto one pipe.
Align other pipe and slide coupling on to it, up to the mark.
Have hammer and small board handy so you can tap it into place if it
gets stuck.

May have to order Hercules glue over internet, Home Depot no longer carries it.

Dooreo,
I suggest installing a union as part of your repair sequence. Many PVC piping repairs require the use of primer and glue, and, the glue needs time to cure before the joint becomes watertight. The union will allow you to leave the piping repair unconnected until the glue / solvent has set; the glued joint will cure faster when it is exposed to the atmosphere inside & out. I also suggest visiting the pipe / solvent / glue manufacturers website for information on the best methods for using their products...

You can also use the new PVC lock fittings with out the glue, open the link below and see the YouTubes, also you can use the PVC-lock slide repair fitting or PVC flex-coupling see the images below,Thanks.


John

https://www.youtube.com/watch?v=8Db8jc8DMcU


https://www.youtube.com/watch?v=ncE5d9IZzAc