VW transporter t4 no power assist on breaks

[HP_12345]

Hi I have a t4 P reg transporter which I have had for years now. I bought it 2nd hand and immediately I noticed the breaks were hard when you first started the van. Clearly the vacuum was not working for the power assist. They cut in after a few mins or high reving or if you go around the corner in the van. I have had 3 new vacuum pumps, a new server and the main VW dealer spent about 8 hrs on it and basically said that they would have to rebuild the engine if I wanted to fix it. It is worse in cold weather and really does need to engage the gears for the vacuum to start - ie you can't just leave it on for 10 mins for the vacuum to start. The main dealer did try something with the cambelt and basically thought it might be the oil pump.
If you pump the breaks continuously you can cause the breaks to go hard but you have to pump fairly hard to do this and be stationary or the vacuum remains.
Anyone had a similar problem or got any ideas.
Appreciated any thoughts ?