Technical 1956 chevy master cylinder issues

I am redoing the brake system on my 1956 chevy sedan. I bought a new factory style master cylinder from napa which immediately leaked out the rear seal when I started to bench bleed it. Ordered another and waited a week and a half for it to come in and it leaks too. I am wondering if they have been sitting on the shelf too long and the seals have dried out and shrunk? Is there something I can soak the seals in to get them to expand or are these aftermarket master cylinders junk right out of the box? They look like cheap junk compared to the original and I am contemplating trying a remanufactured one or rebuilding the original. Any thoughts/tips/tricks are appreciated. Im not interested in a dual master/power discs ect, I just want to make the factory braking system work as it should.

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When you a bench bleeding the cylinder, are you pushing the piston all the way to bottom of the bore?
If so you may be pushing the lip seal past a hole it normal doesn't and tear it.

 

And if you still don't like the new one, pick up a honing stone, clean up the bore on the old one if it's not too pitted, and install some new rubber cups.

 

I've seen the later '62-66-ish masters swapped in to the tri-5 cars. I would go with that unless you are stuck with using the OG design for some reason.

 

I feel your pain 66gmc. I got tired of painting my firewall every month because of ****ty replacement parts. Got my original master sleeved and rebuilt, and haven't had a problem since!

 

Thanks for the replys. I was pushing the piston about an inch in, it started dumping fluid out the back on the second stroke. I ripped it apart last night and the seals looked ok so I re***embled it and tried bleeding it again and it held fluid. Tomarrow I'll be bleeding the brakes and find out for sure.
Thats good to know about the later chevy master cylinders! It looks like I can get them locally for much cheaper than the junk tri five unit, now I wish I had known that sooner!
In the long term Im thinking it may be wise to invest in a rebuild kit for the original master, but for now I'd really like to just get this stupid car on the road!

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The later one I mentioned holds more fluid, another bonus. Just set the rod length the same as the old one and you should be good to go.