On a drum brake the shoe return springs pull the cylinder pistons back after pressure is released from the wheel cylinder. What causes disc brake pistons to return?

I have a front brake that is dragging and causing rotor heating and vibration and I am sure it is because the piston is not returning home. But I don't know what causes this or how to correct it.

I know that the flex lines are all brand new and that the disc piston was recently rebuilt.

From the sounds of it you might mave a caliper locking up. I would say it's either time to rebuild it or replace it.

The primary cause for dragging front discs is a sticking caliper piston. The solution to this a rebuilt or new caliper.
Another cause is dirt and corrosion on the sliding mechanism which does not allow the caliper to retract as it should. Cleaning the sliding points and replacing the dust shield s can sometimes cure this problem.
The most unlikely but possible is a bad brake hose. These hose deteriorate on the inside and once in a while develop sort of a flapper valve inside. That is a piece of the hose wall tears partially free and on occassion blocks the return flow of fluid thus causing the caliper to drag. It sometimes bleeds back down over a period of time so it very hard to detect.

Are you running a master cylinder intended for disk brakes or drum brakes? If the latter, it has a residual pressure valve that maintains about 10psi on the system. This residual pressure is necessary in drum brakes but causes brake drag in disk brake systems. The valve can be removed.

To answer the original question, the rubber seal on the caliper piston is what returns the piston. (analagous to return spring in drum brake system) Yup, that's all.

Agree with all other responses- caliper piston could be sticking and in need of rebuild.

As far as a problem with a pre-existing residual pressure valve, wouldn't that cause both brakes to drag not just one? Even though the flex hoses are new, maybe there is some crud in them blocking return flow of brake fluid?

The master cyl is the same as was used in the donor of the caliper, so I assume it is correct. This problem just started happening recently and did not occur for a year prior, so I assume that a residual valve is not the problem.. I consider that the hose or dirt situation is remote since the I did the bleed and it was a clean job and I replaced all rubber hoses at the same time.

I will check the slider for problems...this is a possebility.

In the event that this does not cure the problem, how does one go about rebuilding a caliper or does it just make more sense to buy a rebuilt unit? This is a caliper from a '69 Camaro, I don't even know if they are available as rebuilt now.

HEY F1; I had the exact same problem on my 38 Lafayette street rod. I took the caliper off, extended the piston all the way out, pushed the rubber boot away. It had brake dust fine like powder in there. I cleaned it with WD 40, coated piston with vasoline. No problem since.... GOOD LUCK....DAVE

On the rebuilt caliper- 69 camaro should be a single piston. Common to late 60's early 70's mid size GM cars. Should be available from parts store. The are popular in circle track racing. Speedway motors has rebuilt calipers in their catalog...