Couple of things come to mind.
So they work well forward but not in reverse?
The master cylinder your useing is for a car with disk front drum rear? Hope its not one from a car with 4 wheel disks because your back brakes wont work. One of the outlets, the one going to your drums must have a one way valve fitted to it. It usually on the master cly just under the drum brake pipe fitting, inside the master. This is to stop the drum brake springs bringing in the shoes too far in, the valve hydrolicly locks the slave cly in your drums. If you dont have one, when you press your pedal the front works but the back will not contact with the drum because they have returned too far. If you have it fitted to your front the disk brakes will drag because they cant return.
Your rear brake shoes are fitted the right way around? One is usually a lead shoe and the other a trail on most cars. It will effect the way the brakes work forward compared to reverse.
The last that i learned the hard way after converting a car to disks front and larger drum rear (at high speed i must add), is that your brake proportioning may need to be adjusted. Its a valve that can be added to limit the amount of braking to the rear.
A good test is to brake HARD a couple of times to see if the front or rear locks first and then play with the proportioning to get even braking in an emergency.
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