Outer Limits 4x4 Board

I recently purchased a 2007 stx navara 2.5 diesel , On a few occasions when high reving and wheel spinning on muddy hills I have clutched and braked at the same time only to find a stiff brake pedal, which causes the vehicle to roll backwards untill I pump the brake to get it to stop.
I have rang the nissan dealer to tell them the problem to be told the clutch is vaccum operated and the brakes are vaccum operated and at high revs the vaccum pressure drops to result in a hard brake pedal ,too bad nothing can be done about it he said.
Can anyone offer some advise.
Thank you Troy

Clutch or brake?

Brakes have a vacuum reservoir, it would take a lot of braking to lose all that vacuum.

vacuum assited brakes ??

you might want to try asking here http://www.nissan-navara.net/index.php as there is a few dedicted nissan repair people there.

Could put in a vacuum reservoir tank like in the Rover Maxi-Drive diff lock setup?

Lucky I haven't had my Navara in your situation!