Drum Brake Issues

[Janz99]

Hey guys, I have a 68' that has all four drum brakes and a relatively new booster/master cylinder (previous owner installed).

I have been daily driving this truck for the past couple of summers and have never had brake issues. A few weekends ago my son and I took the truck out camping and on our way I noticed the brakes were dragging slightly.

By the time we got to our camp site the brakes had basically seized on and two of the four drums were smoking. I couldn't tell if all four of the drums were seized or just the two smoking drums as I didn't have a jack with me at the time.

I let the truck cool down for about an hour and was ready to call a flat bed to tow it home. Decided to start the truck and see if I could move it and luckily the brakes had let go.Drove the truck around for the rest of the weekend and for the following 3 weeks without it happening again.

Last night on my way home just as I was about to pull into my garage, I noticed the brakes were starting to drag again. I was able to park the truck but again I was short on time and wasn't able to confirm what wheels were locking up.

My question is, since its a drum/drum setup, is there any sort of valving inside the distribution block that could cause this, or is this strictly a master issue? I think I can rule out wheel cylinders due to the fact that it happens to more than one wheel at a time? Any suggestions on where to start the trouble shooting the issue would be greatly appreciated.