Do you have a manual and did you follow the sequence. Have you bleed all wheels. Can you answer the questions and my previuos psots. Regards Peter

I have a horrible feeling that if using two people, the person who is at the nipple is not closing it down soon enough and air is re-entering the brakes at the nipple.. Are you using a tube with a non return valve?

Before you worked on the brakes the master cylinder piston was being pushed down probably just 1/3 of the master cylinder bore, so this part of the cylinder gets polished up & worn into the seal on the piston.

When you push the pedal to the floor during bleeding, the master cylinder piston with its seal get pushed down the last 2/3 of the bore where they havn't been before. In an old car this can rip the seal lips & so you loose your pressure.

Get a pressure bleeder & bleed again according to the instructions. If you still then cannot get a firm pedal suggest you get a new master cylinder.

You have a post in discusion about this, dont you read it

you really should get a one way valve bleeder its easier,as someone said I suspect you have put air back in the system while bleeding,also when the person on the pedal pushes pedal to floor its held there untill you lock off the bleeder then the pedal is released I bet thats where your going wrong

Try clamping the flexi pipe/s on each side then press the pedal to check if its solid if it is you still have a problem with the rears,if its not solid then its another fault possibly the master cylinder

Its also possible the bleeder valve isnt seating properly allowing the air back in even though it seems to seal,other wise check out the wheel cylinder seals to see whether they are leaking?