Spongy brake question

[Miztaziggy]

Had some strange A View from the brakes at Silverstone today and wondered if anyone can point me in the right direction.

Basically at the start of every session before the brakes got hot they felt very spongy and a lever would pull back to the bar just about.

Going into the first corner they felt awful I could pull the lever right back and there wasn't much braking Force.

By the 3rd of 4th corner they were much firmer and the breaking was much better. By the end of the session they were spot on though they were they were squealing a little bit.

I've heard of brake fade when the fluid gets hot in the air boils. This is almost the exact opposite and I have no idea what could cause it???

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[AMCQ46]

A bit of a guess but perhaps the master cylinder isn't being allowed to open far enough to let new fluid in. Check that the piston is coming back far enough and there is a bit of clearance between the lever and the piston

[Miztaziggy]

Ahh right yeah that does make sense. I guess the fluid getting warm forces it through the smaller gap.

It used to have a K5 master cylinder on it, but took it to a guy for a major service this year and he changed it for a proper K4 one, second hand I think.
I noticed the brake lever / master cylinder like creaking / cracking noise as if it was stiff yesterday, so I bet that's it.

Cheers Al