Brake problem, any advice??

Guys
I've got a bit of a problem, anybody had similar issues?
I picked up an 01 RSV a few weeks ago, brakes were a bit spongy but came back following a bit of a bleed, since I have been regularly cable tying the lever and bleeding the master cylinder to keep it firm.
Yesterday I had the front wheel off to fit a new tyre, at the same time I freed off the discs so they could float a little. When I refitted the wheel the brake lever came almost back to the bar (even after pumping). So I set about bleeding again, refilled the reservoir 6-7 times, there was some air, and the lever firmed up, but then shuffling the bike backwards and forwards the lever went slack again, still more bleeding....
Went for a test ride and the lever comes back to the bar before the brakes engage, then they are good and firm until the next application, sort of like it's taking one pull to get the pads up to the disc and then when they are there the lever is fine.
Pads aren't great but have a little wear left in them.
Any words of wisdom greatly appreciated.
Ta

Thanks, I was thinking I ought to change the pads to eliminate them, the ones in there are Brembo, is that the default choice or are there better alternatives?
In terms of fluid I've been using dot 4, A) Because that's what I had in the shed. And B) Some of the stuff I've read say Dot 4 mixed with 5.1 makes jelly!
Although 5.1 may work better at high temperatures I don't think that is the problem as I haven't managed to get past the front gate yet!

Lees
I think you may have it, I was starting to wonder whether the master cylinder could be sucking in air, have just bled through 0.5 litre of fluid and still getting air bubbles, then when I shunt the bike about to test the brake it goes spongy on full right lock!
Anywhere else in the system is under pressure so you would expect to see a fluid leak, but of course it isn't under pressure on the reservoir side.
Here's hoping, off to find some pipe now.

Looks like it might be the plastic union betwwen the master cylinder and the rubber pipe, doesn't seem to be threaded anybody know how it comes out?

Looks like it, I've just found a picture of one,they're available for the grand sum of £3.50

Yep, that was the one all the air came out of....because it seems to be going in through the hose union as quick as I can bleed it out!