My car is an 85 and has this style proportioning valve. Can the spring upgrade be done to this style also? The plug is on the bottom not the front like the square style valve. I do know the lines are metric like the newer style valve.

 

I do not know but it might be worth swapping to the new style valve...

 

This is true, you could always switch types. Heck, since it's all out you could install and adjustable style.

Check the thread on the cheap-proportioning-valve-upgrade thread. There has been different styles discussed. IIRC, the answer is yes. However, you may want to review the Calculating Brake Torque thread once you identify your components to help you decide which spring you want to use.

 

This is a pic I found online. Everything is still in the car. I do have a parts car that has the newer style valve in it. I was just trying to save myself some time.

 

You should be able to upgrade that style. It still works on the same principle, you may just need a different diameter spring.

 

I have the same prop valve in my 85 iroc. I put a tilton prop valve after this one in line, use them both, Works great.

 

Does anyone know why the larger line from the master cylinder feeds the rear brakes? This is opposite from every other car or truck I've seen. Typically the larger line feeds the front brakes since the front brakes are larger and have 2 separate lines running from the combination/prop valve. I'm sure there's a reason for this but I can't figure it out.

 

I think the larger rear line is carryover design from the drum brakes. Not needed for disc brake but doesn't hurt anything either. That's the rumor I perpetuate anyway.