It sounds as if you have replaced everything but the booster itself. The statement the brakes feel fine with engine off should be telling you something too. Engine off, the pedal should be hard, and depress very little. Pump the brakes a few times, engine off, to relieve any vacuum. Then hold the brake pedal down, and start the car. The pedal should drop some. I personally would try to locate a stock booster first. I believe this is where the real problem lies. You do have the check valve installed in the vacuum line going to the booster, I presume, and it is working properly.
The original pushrod is adjustable, but if you used the OEM M/C and booster as is, there should not be an issue. My concern is that just replacing parts is not discovering the real problem. You could go to manual brakes, but you would need to reduce the M/C bore to around 13/16 or 7/8 (max), or else you will need monster thighs, and a well anchored seat to stop the car. With power boost, 1", 1 1/16", or 1 1/8" will work.
Does the car have front disk as well? I dont remember if the car was OEM disk/drum combo in 68. My memory isnt THAT great, lol.
How did you determine the pedal was returning slow? Do the brakes drag when slowing down, and you release pedal? Is/was there a spring on the brake pedal, under the dash?

 

One other thing I might suggest. With engine running, car parked, press the pedal hard, then let up. Does it return slowly? If so, with engine running, depress the pedal, and have someone unplug the vac hose to the booster, and let up on pedal. Does it still return slowly?
When you bled the brakes, did you have the engine running, and 2 people? or did you use a vac pump at each wheel? If with 2 people, was slow pedal return observed after opening bleeder screw and closing?

 

we have bled the brakes a ton of times, but it has always been with one of us pushing on the pedal with the car turned off.
When driving the car with the vacuum on, the brake pedal goes all the way to the ground and you have to pull it back up with your toe to get it back up fast enough.

I will be trying a new booster tomorrow. This may solve the problem. We'll see. If it doesn't, I'll be letting you guys know.

Thanks for all the help Dave!

 

Once the brakes are properly bled, your pedal should never go to the floor, even during panic stops. Since yours goes to the floor, you still have air in the system. When you bought the new MC did you bench bleed it before installation? That is commonly overlooked and it complicates the bleeding process. I still think your booster is bad or installed wrong but you have bleed issues too.

 

I agree. This info wasnt provided to begin with. Again, you do have the check valve installed in the vacuum line to the booster?

 

If you DID bleed everything correctly and are absolutely positive that the brake lines are hooked up to the proper reservoirs on the master cylinder, you could just have a bad master cylinder that is bypassing internally on you.