A brake proportioning valve is designed to reduce the brake line pressure to compensate for varying road &/or load conditions. So for fitment of one to the rear line you'd want to be sure that the car was overbraked on the rear. If your fronts that you plan to fit  were too powerful then the valve would be useless, you can't dial up more line pressure than you produce, that's my understanding of the how the accessory ones  work. That's the beauty of a bias pedal box,  you can change the balance of pressures front to rear, not just only reduce the line pressure as the proportioning valve does.
 
A properly designed pedal box with the master cylinders mounted in the car above the pedals has a different lever ratio to that of a standard pedal box  so the pedal effort is much less, even with un-boosted bias boxes, when compared to those applications where people just bolt a master cylinder direct to the firewall and use the standard pedal/rod geometry and no booster. If  you are concerned about going without a booster because of your bung leg, pedal effort is also unlike standard brakes with a faulty booster!
 
SFE 

I agree. My knowledge is that a proportioning valve is designed to moderate the line pressure to the rear brakes. Some have a "knee" point to limit the pressure as braking forces increase to prevent lockups as the weight distribution moves forward. To me this is the opposite of what you're after as disc brakes require more pressure for a given deceleration rate.

Pauls post leads me back to comments about the function of the proportioning valve in the engine bay, my understanding is that it allots more pressure to the front ( discs ) and less to the rear ( drums ) in the standard setup. I have discs on the rear- smaller than the front ones, do I leave the proportioning valve as is?   Maybe I need to look at plumbing in a late model version from a 4 wheel disc setup- like the Focus my rear brakes came from? Len

I wasn't aware that there was a proportioning valve on a standard Escort? I know of the pressure differential valve with the switch bet where is the proportioning valve?

 The 'proportioning' of pressure is usually done inside the master cylinder, that's why there are separate  front and rear lines exiting it. People keep referring to the device on the RH inner guard under the booster bracket as a proportioning valve but it's not. It is the brake failure switch, a simple pendulum switch, one end is front line pressure in & out, the other end rear in & out. Lose front or rear circuit pressure and your failure light comes on as the pendulum moves and triggers the switch. You will note in your workshop manuals it is actually called a Pressure Differential Warning switch
SFE 

Yep. Thanks for clarifying. Due to the difference in line pressures between front and rear as I'm going with a bias pedal box I can't use this switch. Instead I need to run a fluid level sensor on the reservoir to satisfy my engineer. Off topic sorry ....

Here in Victoria I run a non escort car with bias pedal box with F & R lines plumbed through a mk2 Escort failure block to 4 wheel discs, satisfying the vicroads approved engineer and have had no faults. See my posting earlier on in this thread above. It does still  work. Also had a mk2 Escort with 4 wheel discs & bias box plumbed this way
 
sfe 

Fyi : Minis also ran a remote booster. Google vh40 or vh44, looks like pbr still make remote boosters

Thanks SFE, for your explanation- now makes much more sense- been told a number of different things about this block in the engine bay. Len