Hi,
I purchased a Forge Dump valve from somebody many years previous and I was told it was working. I never fitted it to any car since and I was thinking of selling it now.
How do I test if this Dump valve works without fitting it to a car. Any help would be appreciated.

 

if you have a vacuum pump you can test it to see if it moves the piston (connect to small connector). It should open it a bit, you should also be able to move the piston with your finger in the bigger hole. (any Air conditioning engineer / installer will have a vacuum pump

If you have access to a compressor you can test the positive side by letting it blow some pressure off.