I am having toruble locating a proportioning valve for the rear wheel brakes on my 1994 Cadillac Deville.?

I have checked all our local salvage yards nothing since Pres. crushed all older cars. why should I let this car go when I get 28 MPG on the open road. It's just this valve is leaking brake fluid. Can you help me? My mechanic and I are running out of ideas....

Sandi NIven