This is the valve and rebuilding kit from Moss with the instructions.

So let's get it apart and see what's lurking inside after thirty plus years.

By the way, that white switch is available from Ford Motor Company and
the part number is #C8AZ-2B264-A.  It doesn't matter which terminal that you
install the single wire on.  It only acts as a ground and you'll see why in a minute.

The good news is that it is really pretty clean in there.  I've seen much worse.

The plug and gasket came right out and no stripped threads, so that's good too.

A little prodding with a dental pic helped the piston to come right out.
As I said earlier, the brake fluid wasn't bad in this car and it's not packed with rust.

So that's everything, except for the copper gasket on the plug that fits on the housing.

For scale purposes these parts are really big compared to my mini pliers.
 

I've cleaned the crap off of the old valve with brake
fluid and then polished the valve with Never Dull. 

One VERY IMPORTANT item to remember:

Please note that there are two (2) different seals sizes.  Only one size will fit properly.
It's your responsibility to choose the proper size, not mine or Moss Motors.
Remove the old ones and compare.  It WILL be obvious to you which one is right.

Just in case it isn't so obvious, please take 30 seconds to read the very clear instructions.

If you take your time, you can polish this nicely and insure that no burrs tear the seals.

Please note as I will refer to this at the end, the very center of this piston is where the
plunger shaft of the seal will fit and will actually NEVER make contact unless the piston
moves back and forth allowing it to ride up the vertical ramps on either side of center.

Once it reaches the upper part of it's travel, contact is made with the internal top
connectors of the switch, The circuit will be complete and the light will come on.

For cleaning the inside of the valve, good old fashioned pipe cleaners work
very well and will not harm or scratch the brass valve in any way.
 

Doubled over and soaked with brake clean and the brake fluid, they
will leave the valve body spotless inside and ready for the piston and it's new seals.
 

For the smaller outlet sides the cleaners work well as singles.

That's the way that it should look when you're done with the pipe cleaners,
except for the sealing surface for the copper gasket under the big plug.

A little polishing with some 1200 wet paper and some Never Dull down inside gets
everything looking like new again.   It's about ready to go back together again.

I chose to clean the valve body with Never Dull and then Brasso before final assembly.

If you do get one of these used, make sure that the threads are OK before starting.

Put a little brake fluid on the valve and inside the bore and slide the piston down
inside the valve body.  It should fit snuggly and move smoothly.  If you note the
bottom of the switch pictured above, you will see a shaft that must fit down in
between the two larger sides of the piston when it is centered properly in the
valve body.  It will again be obvious from above as you slide the piston in place.

There it is, already to go.   If you are smarter than me, you will have removed
the polishing residue of the Brasso and will have already clear coated this by now.

Just remember one that that many seem to forget.  If you have brake fluid leaking
from the switch, replacing the switch will NOT fix the leaking problem.

As you have seen the two seals on the piston only serve to hold back the fluid
in each respective side of the four (4) circuits to each corner of the chassis
and once there is a leak in a line or wheel cylinder or caliper, the valve can move
in the direction of the leak or weaker pressure side and will cause the shaft on
the bottom of the switch to make contact and cause the brake warning light
to come on as a warning that pressure has drop when the pedal is applied.

If the seals have become cracked / hardened and fluid starts to go by them,
it has only one avenue of escape and that is up through the moveable plunger
shaft of the switch, which is not sealed to resist the pressure generated when
the brakes are applied.  This is why replacing the switch will solve nothing.