[Walt Fricke]

Old rubber brake hoses at the wheels are a traditional suspect. It appears the lining can split and act like a sort of one way valve. But since the hose is multi-layer, it doesn't leak.

However, pulling back on the brake pedal with your foot wouldn't really cure this situation, and typically this one way valve thing you would expect to affect only one caliper, at least at first.

When the brake peal is fully retracted, the holes in the master cylinder, which connect the fluid reservoir to the internal piston chambers, are open. As a result, the brake fluid and lines are all at atmospheric pressure - and hence exert no force on the pistons in the calipers. When you step on the brake, the pistons move forward, closing off these holes, thus allowing the pedal to create the pressure you need. If the MC piston(s) don't retract fully when your foot comes off the brake, the pressure in the brake lines won't bleed off, at least not fully.

The long rod connecting the pedal to the MC bell crank was adjusted in length so that the piece coming out of the power brake canister has just a little bit of play where it contacts the outermost piston of the MC. This guarantees that the spring inside the MC can fully retract the pistons. However, since your system worked initially after the new MC was installed, this long rod is probably properly adjusted.

If, somehow, the vacuum isn't being entirely released, that could cause a drag. I wouldn't rate that as high on the suspect list, but you could disconnect the vacuum (probably a good idea to plug the hose so the engine isn't affected) and see if this affects the pedal return.

I agree with those who suspect the pedal cluster. If the bushings in the pedal cluster have become gunked up and stiff, that fits your symptoms, even though the pedal return spring is fairly stout. The bushings do wear over time, though usually that manifests itself in sloppiness and sometimes issues with clutch release. A quick test might be to spray penetrating oil into the brake pedal bushings while wiggling the pedal back and forth. You need to remove the floor board to do that, and welcome to the Houdini pretzel club, but lots of us have worked on pedal clusters.

Whether or not this band aid helps, a cluster rebuild is in your future. Pelican doubtless sells a bushing replacement set, either nylon as the original, or bronze which most use for replacement. Plenty of posts on how to deal with the pedal cluster. Our host Wayne's book 101 Projects for your 911 at chapter 40 has an explanation. If there is a problem with the brake bushings, chances are the clutch bushings there are also worn.