A General Brake Question.......

By "drag" do you mean while braking or after braking? Nonetheless, it's possible that one of your front calipers is seizing, giving you irregular front braking.

As far as the leaky rear cylinder, if it leaks enough your proportiong valve will kick over leaving you with just front brakes.

The travel of the pedal is usually M/C which you say is fine, so that only leaves air in the lines. Give them a good bleed.

If you replace the rear cylinders, be sure to do both or else the new one will cause that side to lock up as it will function far better than the old one.

One of your problems is that you've put 16" wheels on a brake system designed for 14" wheels. If you don't think it makes any difference, swap 14's on to it and see. Also, I see you have Granada brakes. Did that include the M/Cyl? Check your pushrod length from the pedal to the booster diaphragm. A rod too long will minimize the power assist.

I'm still thinking the rod length from the pedal to the booster is too long. Did you compare them when you swapped? Is the booster depth and diameter the same as what you replaced? Didn't the Granada have the small diameter booster and the Cougar the large diameter? Sorry for the questions but I'm not very familiar with the swap. Just trying to help with the diagnosis.