[HO455]

It is quite likely your new proportioning valve is bad. I'd wager there is a bit of machining gone wrong inside. Time to call the company you got it from and ask for a refund.
As previously stated there really isn't many reasons to replace a proportioning valve. When you go get a complete brake service at a brake shop they don't replace them. They replace everything thing else that could conceivably fail out of liability concerns. Proportioning valves are extremely reliable, especially for simple ones like ours.
I would reinstall the old one and see if you can bleed the rears with it. Then I would return the new one and never look back.