Disc Brake Conversion Failing

Pull the rear drums and inspect of leaks and foreign material
on the brake shoes.

Some of the Masters look similar BUT have different front to rear
proportions built into them
IE: 61 to 39 or 69 to 31 are two I remember.

I use 61 to 39 that was a Mopar Perf part.

Also make sure residual pressure valve is in correct place on cylinder

That being said my son does all our brakes as I am no expert on them!