The manual front disc brakes on my Challenger seem to be dragging. I believe these are a Master Power conversion kit.

Had the front end up off the ground, and while I could turn the wheels, they won’t spin. There is definitely a lot of drag.

Pedal is coming up all the way, and I even shortened the rod a bit to see if it needed to come up further (it didn't).

What might be causing this issue? And how do I fix it?

residual valve in mc .. take it out , leave rear in , Are rear discs ?
Also check pushrod length .
Adjust brake swith on pedal assembly , may not
be retracting all the way ..

First, establish if it is residual brake line pressure or something mechanical with the calipers/sliders by cracking open the bleeder.

If cracking the bleeder relieves the drag, pursue kinked or failed line, stuck prop valve, messed up MC, or maladjusted MC rod.

If it doesn't, pursue/clean/lube the workings of how the calipers slide.

I'm getting the same drag on both sides, which leads me to believe the problem is well upstream.

Shouldn't be a residual valve on the front, should there?

There is nothing restricting upward travel of the pedal. That's the first thing I checked by removing the brake lamp switch and shortening the rod.

to check the residual valve, take the line off the master cylinder and look in the port. There'll be a hole in the middle tube seat. Stick a toothpick in the hole and see if you can feel something 'springy'.

If you can, that's the pressure valve, so you'll need to use an easy out to remove the tube seat to get access to the pressure valve.

Or, it could be like my master cylinder when I went from drum to disc, and just didn't have the RPV.

Some drag is normal. They won't spin free like drums do.

I'm familiar with the normal drag. This is more drag than I've felt on any other disc brake car.

I can usually spin the wheel a little. Not so in this case. It stops immediately.

Yes it's a judgement call as some drag more than others and I had one (oe) that seemed too tight and a rebuilt caliper took care of it. I'd crack the line at the caliper to see (in seconds) if it is a psi issue or a mechanical one and most likely could crack it a bit and not have to rebleed

Yep. Tonight I'll check to see if there is pressure on it.

You might want to check and see how tight the wheel bearings were adjusted while you're at it.

Last time my car did that, it was the brake fluid dist block ( prop valve )

Do They eventually free Up ???????????????????????????????? When u come back to it ten minutes later ??

Some drag more than others.

If they aren't heating up while your driving it, or pulling, then you are chasing a ghost

Rechecked it last night. RH side felt pretty normal. LH side definitely has more drag. Don't feel it going down the road though.

Need to pull the wheel for a closer look.