Question bleeding brakes w/ prop valve

I am trying to bleed the brakes on the Belvedere and I am having issues getting a firm pedal and air out of the rear lines. I have a GM style factory disc brake proportioning valve on the car. I have noticed tools sold online to lock the valve in place while bleeding. Do I have to have one of these tools, or is there another way to keep the valve from setting under low pressure?

Thanks!

Never mind. Googled the procedure and found what I need to know.

Thanks!