Seem to have a problem that has me completely beaten.
When I’m free wheeling down a hill everything is fine with the drivetrain, but as soon as I brake hard my rear mech comes forward and the chain goes really slack and tries to fall off. I thought it was a stiff cassette but surely if it was, the chain would go slack when I was just coasting?
Anybody have any ideas?

What frame?

Bust tension spring in the mech? Possibly combined with a sticky freewheel that rotates forward with the wheel?

Suspension or HT bike?

mech coming forwards would be due to chain pulling tighter, surely – unless you were braking so hard that the mech’s own inertia pulled it forward but that’s not going to happen

Brake squat or jack and/or simple weight transfer of the rider is my guess, depending on how you position yourself when braking hard and assuming it’s a FS bike – either will unsettle the chain a bit, though one initially slack and the other initially tight. (or something)

Chain caught up in brake?

… or is your hard braking just a sign that you’re on rougher ground and it’s bouncing off ?

Oh, and JCL – ??

I’m gonna go with sticky jockey wheels and you’re back-pedalling a little when you brake.

Take the chain off, do the jockey wheels spin freely?

Second culprit, what about the freehub, maybe needs a service.

Mate of mine has this problem with a demo bike and it turned out to be the plastic disc between the cassette and the spokes.
It has become loose and was jamming the cassette stopping it freewheeling.

So, I reckon something is causing your cassette to not freewheel correctly.
Our sticky jockey wheels.

Yeti 575 frame.
I have bust the tension spring on the clutch for the rear mech but I presumed you could just turn that off anyway and it would act like a normal rear mech? Must be wrong then. I’m now presuming this is the cause along with maybe a slightly stiff cassette?

Mavic hub ?

Chain going slack is usually a sticky freewheel but why it’d be doing it when braking but not when coasting is a mystery.
What happens when you back pedal?

unless you are locking the wheel up or pedalling when braking, then you will be freewheeling still. it’ll be a knackered/gunked up freehub.

Maybe I’ll have a look at the freewheel too while I’m at it then.
Seems fine when I’m back pedaling.
American Classic hub not Mavic.
Maybe I’ve overtightened something?
Bust clutch spring on mech probably not helping much. Chain came off twice last night.