While the specifications of the proportioning valve you have should be correct, if it is not working for you, consider removing it and install an adjustable valve.

Before you do that, do a thorough rear drum inspection and check for sticking wheel cylinders, worn out backing plates, worn out or stuck self-adjusters, oil or grease on the shoes and see if the primary shoes are in the right location.  When I got my 69 disc-drum car, one side had the shoes on backwards and it would skid if you tapped the brakes.
Then, it check the fronts. If a caliper piston is seized, a caliper slide is seized, or a hose has collapsed, the rear brakes will do all the work and tend to lockup before the fronts do anything. Even loose bearings cab cause ineffective brakes.
Once you determine the brakes themselves are all working properly, then consider a new prop valve.

Well it looks like the drum brakes were the issue. Here’s some before and after photos to show. It’s been quite a while since I’ve done drums. Man what a PIA lol. Only got driver side done last night. Today will be on to passenger side. 

I'm not a drum brake guru, but I put the small shoe on the front.

Doh!   Thanks guys. Like I said it has been a while. When putting them on I compared the shape of all the notches and holes. But missed the shorter pad vs the bigger. Thanks.

Alright. Was able to get it corrected. Thanks again for pointing that out. 

If you want to know why the leading shoe doesn't have to be as large as the trailing shoe, this video explains it.  It's an old video but the information is there.  The key words are "self energizing brakes".  It is the rear shoe that does the braking.