'68 Coronet, Reproduction tandem booster & large bore (1.125) factory style reproduction master cylinder. SSBC front disk conversion.

When bench bleeding the MC, the piston retracts easily, but stops around .400" short. Over the course of 45 seconds it slowly retracts to its final position with the end of the piston protruding .090" past the flange.

Isn't it supposed to easily retract all the way without a hang-up? This style of MC has a screw from the underside that holds the rear piston in place, as opposed to having a c-clip as the positive stop at the end of the bore.

Someone recently posted those master cylinders have powder-coating residue trapped in the fluid ports, preventing the piston from retracting.

I would pull the brass seats out with a screw and claw hammer and clean the passages underneath.

Wow, thanks. I wouldn't have guessed that.

Here is a thread on the subject:

http://www.forbbodiesonly.com/moparforum/showthread.php?89498-New-Front-Disc-Brakes-Drag/page2

I also recently sold one of those master cylinders that had the same problem.

That's a very informative thread! Thanks. I thought I flushed out the ports really well with brake cleaner prior to installing the piston assemblies (which I soaked in brake fluid and carefully inserted into the cylinder).
However, this time I'll spray into the ports from both directions to dislodge small contaminants and make sure the residual valve is clean before putting it back in.

The Ram Man's video to disassemble & re-assemble the MC was helpful. He has a special tool to pull out the front piston assembly. I guess I'll make one. Apparently using compressed air to remove it ruins the cup seals.
Thanks again for your help.

Where can I buy just the brass seats for my 5621 reproduction master cylinder?
I've already bought one $25 rebuild kit for this 1-year old MC. I need to clean out the MC ports again, but I'm not sure if the front piston will return properly after another disassembly & re-assembly.
I believe the residual valve is the correct size for the seat. However, the bleed holes look different in the pistons that originally came with this MC and the Raybestos rebuild kit, and those look different from the factory pistons shown in The Ram Man's video. Based on the size and number of bleed holes, it seems as if these 3 piston sets would bleed at different rates.
I'm supposed to drive the car on a 3 hour trip in a couple weeks, and hoping to figure this out soon. I appreciate it.