1968 C10 4 wheel drum brakes are spongey

[HeavyHauler]

The manual four wheel drum brakes on my 1968 C10 were very mushy and would barely stop the truck. I noticed there was alot of what looked like stain from brake fluid on the old really rusty mastercylinder so I decided to replace that for starters. It was rusty inside as well I noticed after I took it off.

I purchased a new mastercylinder and now the brakes are hard but barely stop the truck. What could be the issue with the brakes barely wanting to stop the truck.