I have had this issue on and off for awhile now. My brakes will have a pretty heavy drag after the car has been driven awhile.

This problem seems to be related to master cylinder temp vs actual temp from heavy braking. I originally thought it was a pushrod issue but that is not the case, I can lift the brake pedal some and it has zero impact. When this happens the pedal is more firm than normal.

I idled in line for 20 minutes or so getting into the track last night and the pedal was rock hard by the time I got into the gates, the car cooled for about 20 minutes and I looked over and saw the brake lights on and the pedal had returned to normal feel after a few pumps.

I have the Dr Diff master cylinder kit which is an aluminum mopar unit with the smaller of the two bore options(7/8 I think). Willwood discs front and rear.

Should I toss a new master on it and see if that fixes it or is there something else I am not considering.

The level of drag is enough that if you are rolling in neutral you don't necessarily feel it until it almost comes to a stop then you feel it grab.

I had an issue with the brake fluid spitting out of the "breather" hole in the front cap, master cylinder was warm to the touch but not hot. Never worked out how, as there is a rubber seal for the caps but the fluid was getting past it when hot. I made a shield (as pic) and I put a heat resistant mat on there too to solve the problem. I don't recall the brakes dragging though ??

Had the same problem end of last year on my 71 GTX. Whole brake system is fresh, new calipers, master, hose, etc. only thing reused from the assembly line is all the valving. Had about 500 miles on it then all of a sudden the front brakes started locking up. Cool it down and it was fine. Just getting it out of the garage today and see if the brake genie showed up to fix it. All I can think of is the calipers hanging up, time will tell.

I would change the fluid. If you're DOT 3 then move to a DOT 4plus or DOT 4 super of LV. If you're DOT 5 then flush.

All of them absorb moisture and trap water except DOT 5, The DOT 5 separates the moisture. I suspect the moisture in the system is expanding.

gotta be a bad master or the pushrod , the test is get the master hot so the brakes are dragging & then open the lines at the master & see if the drag goes away , then isolate the problem by getting it hot again & loosen the master bolts to give it some slack, OR if you have an adjustable rod shorten the rod & see if the drag goes away

Check both ports to make sure there is no residual valve in either port by using a small wood screw into the brass adapter (seat) and pull it out to see if the little flapper valves are in their behind the brass adapter fitting, seat, or not ,
If they are in their remove them

What Cab said. I had a 69 Charger car that a friend had raced with the original drum brake master cly but had installed factory 70 disc brakes. I noticed it could not be pushed after it got back to the pits. I changed master cly to a disc brake one ( late model aluminum) and removed those rubber check valves. The next weekend, it picked up a tenth and 2 mph! We could push it then when it was hot.

The only time I heated up an MC - dual MC, Maxie exh manifolds, early B - it responded bylosing braking ability.
Putting a heat shield under the MC fixed that.
I once had a sticking rear caliper on another car after a brake job done by a local shop: the wheel was hot to the touch, and I could smell hot metal.
Neither sounds like the issue here, though I may be misunderstanding the OP.
I've heard that check-valve deal before.