Mine was only getting 1/4 boost, I put a new map sensor on it and also a new oil breather hose that had split and it was all good again.

Check this hose, it's the one that goes from the middle of the cam cover through the inlet manifold and connects to the oil filter housing. If its an early one then the hose is quite plasticky and splits around the corrugated bit. The newer hoses are rubber and much better.