Disc brake fade?

scratching my head here, I'm hoping that it's a simple problem but I've noticed that my rear disc brake seems to fade and not purchase as it should when riding, it locks and stops sharp when not underweight but doesn't seem to grip properly under load (I'm 11 stone) I have bled the system and replaced the pads with new but still the same problem, doesn't seem to be air in the cable as it bleeds clean with no bubbles.......can anyone advise?

Clean the rotors?

and use organic pads

That sounds like a combination of dirty/greasy rotors and or pads that have got some grease/contamination on them.

Disc brake fade is a result of the brakes being used too much resulting in the rotors heating up and the hydraulic fluid boiling. This results in a complete loss of brakes as the levers pull to the bars with no result.

It's unpleasant, I've had it a few times when downhilling on my MTB in the alps.

I'm assuming you are commuting on london/local roads rather than descending the alpe d'Huez.

So it could be the pads, the rotor or perhaps you need to do a 100% hydraulic fluid change rather than just a small bleed to get rid of any air

thanks for your thoughts, the contamination of the pads was my first thought so I changed them for new ones plus I've done a complete bleed with fresh mineral oil (they're Shimano) I've also cleaned the rotor both sides with a clean cloth and Finish Line Speed Degreaser.......still scratching my head as I've done hydraulic brakes before with no problem

If you replace the pads without cleaning the rotors then the pads will just recontaminate.

I had a similar issue with a hope brake a while ago that was suggested could be down to a seal deteriorating.

I bought some shimano brakes instead.

Have you checked if all your pistons are moving freely? I may be just that

Could be that the discs are glazed. If they have a smooth glassy appearance that may be it. Will give the symptoms you’re describing.