In this method of brazing, the melting point of the filler metal ranges from 620 Degree C to 800 Degree C and it is used in instrument making. The method is

Because aluminium has a too low melting point and would melt before brass

It is impossible to braze aluminium with normal brass brazing rods because, aluminium has a too low melting point and would melt before brass.
Melting Temperature of Brass normally 900 Degree Centigrade to 940 Degree Centigrade.
Melting Temperature of aluminium normally around 660.3 Degree Centigrade.

Emery cloth

Emery cloth is used as abrasive materials to clean the steel before brazing.

To clean the metal by dissolving any oxide already present

Flux is used in brazing to clean the metal by dissolving any oxide already present.

slowly

When brazing is completed workpiece should be cooled slowly.