I guess you pushed the pistons all the way back in. I also guess you have not touched the bleed screw because you didn't mention it in your list of what you did.
What might have occurred here is that doing this has damaged one of the seals in the master cylinder or - if the caliper was on its way home - the piston seal in (one of) the calipers.
What you should do now is to ascertain where the loss of pressure is occuring. Pump the pedal to remove any servo vaccuum. Get yourself some brake hose clamps which will not damage the rubber and clamp both front brakes hoses. Now try pushing the pedal and if it goes hard your problem lies in a caliper, if not then your problem is almost certainly a master cylinder seal (or a leak but seems odd to have a leak all of a sudden). Master cylinder... you might be able to get a repair kit, or you might need a whole new one. Depends on your car. Report back if you think it is the master cylinder.
By removing one of the clamps you can find out which caliper/brake is affected. Now check around the caliper and see if any fluid has leaked (although it probably won't have got past the outer dust seal). You should also be able visibly see if the piston is moving out at all to contact the inner pad, i.e if you go and pump the pedal and then get out and the piston hasn't budged well then it is the caliper. I had this with a recon caliper from eurocarparts. Fitted it, pump the pedal.. keep going...errr... get out and piston hasn't moved! Send it back, another duff part from them...
Anyway in your case you would not have the luxury of getting a new part for free but you could try overhauling the caliper. You will be able to get a kit comprising the dust seal and the inner piston seal. Clamp the brake hose, undo the hose to caliper bolt and remove the caliper. Open bleed screw and drain all the fluid out the caliper. Remove the piston (which might be quite difficult) and check out the fit of it in the caliper and the condition of the piston and the bore. If all ok carefully remove old seal with a blunt instrument type thing and dunk the new seal in clean brake fluid and push it into place with your fingers, refit dust seal and push caliper back in making sure it is square and goes in ok until it is in enough and then push back with the tool you are using and refit the hose. Before putting it all back, shove a bit of wood like where the brake pads would go to protect piston and have a go at pushing the pedal - and make sure the piston actually moves this time.
Good luck,
Pete
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