I just went through this on my 1955 1st 6400. I ended up replacing everything front to rear including the Hydro-Vac. The problem I had was identical to yours. I would drive and the pedal would get harder every time I pushed. The rear brakes would eventually be so tight it would stop the truck. What I found was the front brakes were completely non functioning. The rear brakes had gotten so hot the rubber parts had literally melted. Another problem I had was bleeding the rear brakes. What I eventually did was retract the rear shoes all the way to make the space in the four rear wheel cylinders as small as possible. This allowed me to bleed the rear brakes and finally get all the air out of the cylinders. The bleeding took forever and was never good until I did this. Brakes are important, do them properly and the best you can the first time.

Bruce


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