Master cylinder refurbishment issues.2819

I have a new seal kit from Chris to refurbish my brake master cylinder on my 1957 Big Boot.  I have previously replaced all seals and rubbers on the wheel cylinders and this was my final job.  Once I had figured out how to get the master cylinder off it was stripped out and the old rubbers discarded.  The cylinder bore was in perfect condition with no scoring or signs of rust and so the new seals were used and the cylinder replaced in the car and connected to the reservoir.  The front driver’s side brake was bled and the pedal was firm. on checking the fluid level, despite topping up, was lower than I anticipated and gradually dropped until the reservoir was empty. On inspection it appeared the fluid was leaking from the back of the master cylinder.  I dismantled everything again and cleaned each part meticulously  in case anything was trapped under the sealing rubber on the piston. I jury rigged a supply to the cylinder while on the bench and the leaking continued.  After several disassemblies and reassemblies the leak appeared to stop and I remounted the cylinder in the car and bled the brake as before.  To my utter dismay the fluid started to drop into the cavity in the body and out onto the floor.  Has anyone had such an experience or have a solution or suggest a possible cause.  Chris has suggested I bite the bullet and buy a new cylinder but I would like to resolve this problem if possible.