Yes, they do work.
http://store.summitracing.com/default.asp?target=egnsearch.asp&N=115+400304+3015
58&autoview=sku
http://www.mpbrakes.com/mpfaqvalving.htm
http://www.stoptech.com/tech_info/tech_white_papers.shtml
http://robrobinette.com/brake_valve.htm
http://www.mirafiori.com/~courtney/128/tech/tilton_prop_valve.html
http://www.gnttype.org/techarea/brakes/prop.html

they all start at zero pressure reduction, then the prop ramps to
limiting pressure.

It would be a shame to reduce rear braking to match the front.  Something
is wrong; and I hope you really find it.
Could the problem be stated as front brakes that aren't gripping the way
they should?  Does the curve of the front shoes match the curve of the drum?
If the drums have been turned, and the shoes aren't shaped to match, you may
have only a small area of contact, away from the ends of the linings.  That
small area quickly overheats and glazes during break- in.  Once glazed, it
won't wear down easily; and you are stuck with it.
  You can change the shape of the shoe, crudely, with a hammer, on a
concrete floor.  A box of them may have been changed, crudely, by someone
who dropped it, on end, on a concrete floor!  "Real brake shops" cam grind
the linings to match the drums.  The drums could be bell shaped, too.  That
would give the same problem, on one edge of the lining.

 That said,  why not build a real dual master system, rather than go to a
proportioning valve?  There's room for a second master cylinder under the
floor, alongside the original.  Maybe both could be moved to the rear, to
leave more room for linkage.  An adjustable "bias bar" between the two would
allow you to set up the brakes to taste; and give the safety of a dual
system.
                                       Mike M.