1972 duster /6 drums

Hello FABO,
When I push the brakes on the ol Duster, I have to pump once before she gives me pressure. I feel like its gonna be the master cylinder that gets replaced. Before I go and start throwing parts......What say EWE??

manual brakes
slant six
drums all the way round

Also, the dash brake light will come on when on that first pump.
Also, in the rain last week the rears I believe locked up first. I dont usually drive in the rain but yknow.....it happens.

Otherwise no pullin when stopping.......

When I call NAPA, they give me a part number M2066

I have an ol RMC 36221 on the shelf if that will work.

I also have a bendix 11323

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