If the car is jacked up in the air then yes the spring will be loose. Are you not getting any rear brake function? Usually its the holders on the rear flex lines to the calipers rusting until they squeeze the flex lines closed.
There are no new Proportioning Valves except for maybe E-Bay at crazy prices.
If the valve is seized, it will be in the near full flow position.
This valve is very important in a front disc, rear drum equipped car. The rear drums are what they call self energizing, and once activated, tend to bite harder as time goes on. Some claim the ratio is 60 front, 40 back in this dual system.
An all disc system needs less defense against this rear brake lock up situation. The rear pads are puny compared to the front and that goes a long way in preventing this.
Wilwood sells a circular handle controlled valve you can mount usually in the engine compartment for around $50.00, you will need 2.
The valve on 96 and up Taurus looks the same to me but would need some research. The 95 unit has the in tube on the top row while the 96 and up has the in line on the bottom row.
You can search on this forum for people who have replaced the unit with restrictor fittings that lower the available pressure.
All Taurus models in our age group have this valve, so maybe the rust free zones could supply what you want.