Here's a illustration of a Master Cylinder
It's the best example I could find, BUT:
THE INTAKE/RETURN PORT and EQUALIZATION PORT are ***LABELED BACKWARDS***
The big ports (Should be labeled INTAKE/RETURN ***NOT*** EQUALIZATION) make sure the cylinder is full of fluid from the reservoir to be ready for you to step on the brake. The EQUALIZATION PORTS (mis-labeled INTAKE/RETURN) are pinhole-sized. They are there to make sure that no pressure remains in the cylinder when the brakes are NOT applied. I learned about those when one "healed shut" with corrosion and the front disc brakes wouldn't release.
So What? Here's my off-the-wall, out-of-left-field suggestion: If the Booster Push Rod isn't adjusted right, you could be able to bleed the brakes with fluid escaping back into the master cylinder reservoir through the Equalization Ports BUT the Pedal would sink till the EP's were covered.
You have a Transverse Mounted Master Cylinder, right? We had a 1971 B300 window van. It came with front drum brakes that we converted to disc from a wrecked 1973 cargo van. Owned it 20 years and found I had to add a grease fitting to the pivot shaft for the bellcrank that changed motion from fore-and-aft to transverse. In checking the pushrod adjustment, make sure everything is FREE. From the pedal pivot inside, out through that bellcrank assembly.
And for Heaven Sake, change that REAR hose too. I agree they don't fail anywhere near as often, but if it's seen 40 birthdays it needs gone.
Also, inspect all your Steel Tubing for Pinhole Leaks.
Last, when you bleed the brakes, always discard the fluid you bled out. It can pick up tiny air bubbles as it escapes through the bleeders under pressure and becomes compressible. That'll give you a soft pedal if you use that fresh but aerated fluid.
