[lucky1]

My 65 GTO has Power, four wheel disc brakes.
Sometimes when I depress the brake pedal it does not return to FULL height
And results in a very slight drag.
It sounds like it might be coming from the rear brakes.

If I pull the pedal up with my toe the drag stops.

It is slight travel in the pedal to pull it up with Hardly any effort.

What might be causing this ?

Thank you.

[Andretti]

I'm assuming nothing's been done recently to your brake system. Sounds like a master cylinder issue, internally binding. You could try flushing/bleeding the system but replacing or rebuilding the master would be a good choice. I would go after that first as I've never seen a booster cause this sort of issue. The rear brake making a sound is understandable as your master is not releasing the line pressure.

If the booster was recently removed/serviced and the pushrod length set incorrectly, the resulting drag would be constant and not something you could relive by lifting on the pedal.

[stevep]

Sounds like a sticking power booster