67 front "disc brake control valve" rebuild-replace?

My 67 fastback with original front disc brakes tends to lock up the rear wheels under hard braking (it doesn't need to be a very hard stop).  So, I ordered a rebuild kit from NPD for what they call a "proportioning valve."  Not to be confused with the "pressure differential valve" / "warning lamp switch" assembly, I'm talking about the tube-shaped in-line valve that controls pressure to the back brakes only.  After rebuilding the control valve, it now holds the rear brakes on after the pedal is lifted.  Reverse is not as affected, but enough brake pressure stays built up that it is not necessary to use the brake to prevent forward motion with the engine idling and in gear.  Eventually, it goes away, but it is not drivable at present.  Any ideas why this is happening?  The piston inside the valve came out with little trouble (spring was rusted to nothing), and replacing the o-rings was not difficult.  However, I did not attempt to remove the little check ball in the end of the piston as the brass tabs that hold it seemed too fragile to work with without damaging them.  Have I restored one part of its function (spring and o-ring seals) while not allowing the check valve to return the fluid pressure?  
 I have ordered what Superior Mustang (MustangBrakeParts.com) calls a "residual pressure valve" but looks in the picture to match what Ford calls the "disc brake control valve" (term in 1967 shop manual).  Anyone ever used this vendor?  This is the only source I could find that offered this part, so I thought I'd take a chance and see what I get. Any thoughts out there?  

Sounds like the mechanism is not returning fully to its proper alignment. I've never dealt with a 67, the Kelsey Hayes on my 66 started leaking and I got an adjustable one from Summit. It's great to be able to dial in your rears to match tire differences or weight distribution.  

There are some hydraulic parts that are rebuilt and some that are thrown away- Take the "rebuilt" valve out and throw it away. Put in a new valve. I'm surprised you could rebuild it.  Replace it with a like kind of unit(my data shows a combination valve PN 2B257) you do not want a residual pressure valve.jlg  

Thanks  for the responses.  2B257 is shown in the '67 shop manual as the "pressure differential valve," which distributes brake fluid to the front and rear brakes and operates the integral "brake warning lamp switch" when one or the other side loses pressure.  I don't think this is my problem as it does not affect fluid pressure to either front or rear brakes unless one side fails to build or hold pressure (my brake warning lamp works, but is not currently on).  The part in question is the 2B09 "disc brake pressure control valve assembly" located just in front of the rear axle (shown on p.2-13 and 2-22 Ford Shop Manual).  
    To be honest, I'm not sure what the control valve does.  I was under the impression that residual pressure was maintained by a valve in the master cylinder, making a separate residual pressure valve unnecessary.  Ford describes the 2B09 control valve (p.2-13): "provides balanced braking between the front and rear brakes under a wide variety of braking conditions.  By regulating the bydraulic pressure applied to the rear wheel cylinders, the valve limits rear braking action when high pressures are required at the front brakes.  In this manner, premature rear wheel skid is prevented".
    From this description, the control valve sounds like it limits pressure instead of maintaining residual pressure.  Another post suggests an aftermarket adjustable valve.  To do this, should I "gut" the in-line factory valve so as to make it just a junction connecting to the rear lines?  

I see the confusion. The 2B257 is really a "proportioning valve"-one which limits the amount of fluid volume from front and rear for the brake system. The "combo" part of the valve comes from the addition of the brake switch which biases a shuttle in the direction of a loss of system pressure if it occurs in either front or rear system.

Some master cylinders have internal valving in them to maintain some internal system "residual pressure".

You may not like the effect of gutting the existing valve on an emergency stop situation.
Here is a pretty good discussion that simplifies how it all works( may need to copy into your browser):
http://www.careersnet.org/automotive/WWCC/Hydraulic%20Brake%20Control%20Valves.ppt#256,1,Hydraulic Brake Control Valves