Hard brake pedal when cold
 

Hi,

I have a 82 diesel vanagon and I've been having problem with the power
break system. When the engine is cold, I get absolutely no assistance
from the power break, so it's quite hard to stop the vehicule. After
driving it for a while, I start getting some assistance and the pedal
gets softer on the driver. However, if I depress the brakes several
times, the brake pedal gets hard again, as if the vacuum pump could not
supply enough vacuum (diesel engine here). Brake booster looks fine
(air thigh) as the brake pedal works properly after the van has been
parked for a few days (assuming the brake were ok when parked of
course).

Any idea on what could cause this?

Thanks

Sam