Brakes on a 68 Shelby

Check for fluid loss, bleed the system, check vacume line to booster.
Tout

 

I suspect the brake master cylinder.
The cylinder inside doesn't seal in one
of the circuits which means you loose braking
power. With this problem you won't loose any fluid
but it may move between the two separate
cylinders in the master cylinder. My bet is that it's
the rear brake part that's not working.
Texas Swede

 

and let us not forget that little item that must be centered in the proportioning Valve. Bleed the Brakes starting with the Passenger rear wheel.

 

It's me again. I meant to say, to check for
fluid moving from one reservoir to the other,
not cylinder. Swinglish from my part. Sorry!
Also when saying cylinder in the first sentence
it can be called piston as well. Hope I got this
correct.
Texas Swede

 

Low pedal: how is the shoe to drum clearance in the rear? Low pedals are almost always the rear brake adjustment with disk/drum systems. Adjust them so there is a little (very little) drag.
Beware: In case the shoes are worn, I'm not sure on the 68's, but, the shoes on the 65/66/67's are a little wider, like from the same year Econolines, get the correct shoes if you need them..