I have posted about this before,
When the rear brake pedal is depressed, you can see the rear master cylinder move away from the pedal. I swear that it moves a good 2 or 3 mm.
I think its the rear bracket that holds the rear master cylinder. Its too weak. That and the fact that its near to the exhaust etc etc.
Now all I have to do is get around to making a decent bracket and testing it.
Either that or my one is a duffer. Saying that though, my rear brake is ok. I bleed it about every 6 months.
Ho hum.