Hoping someone can point me in the right direction... I have replaced my discs and pads and given my calipers a thorough clean up over winter (pistons out and cleaned up, seals checked and regressed with red rubber grease, new oem pad pins and fresh fluid) all put back together and bled with a vac bleeder a couple of times, including the master cylinder. I've got a good firm lever however occasionally I go to put the brakes on and the lever comes almost back to the bar. A couple of squeezes and it's back to a solid lever. Its terrifying when you need some front brake and there's nothing! The mc was rebuilt a couple of years ago and was all working fine before the strip down so whilst I can't rule is out 100% I assume if its duff then it wouldn't build any pressure? I've got no leaks anywhere and everything is torqued up correctly.
What's going on? Could the new pads be catching against the insides of the caliper body and not moving back when I get off the brakes?

Any thoughts welcome!

It sounds more like the pads are coming away from the disc, doing the opposite of your last sentence but for the same reason! If they're tight in the caliper it might be worth giving them a tickle with a file to get rid of paint on the edges in case that's what is sticking.
Are you sure the discs are running true and the wheel bearings are good? They can cause the pads to get pushed back and create a gap to the disc.

Cool. Makes sense. How much 'slop' should there be when the pads are in the calipers? On of the pads does feel a bit tight but worried if I take too much off the ends they'll start to rotate around the pad pin and bind up even more.

Oh, and on your point about trueness, the discs are brand new and there is no vibration when the brakes are working fine, the wheel is straight ( had it done a couple of years ago and 'eye-balled' it when I was pyttibf the front end back together) and the bearings were replaced at the same time and are all spinning freely.

Anyone got any thoughts on how much clearance there should be between the pads and caliper body. Should they'rattle around' a bit or be snug?

I assume they shouldn't be touching the sides?

Just so long as they are not tight,

New or old hoses?

Used briaded lines but all was fine before the callipers were stripped, new discs and pads installed so I think the hoses are a less likely suspect as they've not been tampered with.

Disc standard thickness and pads standard depth?

Discs are thicker than standard 5.5mm I believe. Pads should be standard (benix version of oem pads)