You wrote:
>But with the
Vanagon it's different - if I brake hard I get a rather severe
vibration (the whole bloody van rattles) and the brakes feels like
they grab then slip, grab then slip, grab then slip ... I get a rapid
whop whop whop whop noise and feeling.<

Dan, this sounds like the classic symptoms of warped disks or out of
round drums, although I find it hard to believe a VW mechanic wouldn't
have thought of that before he started replacing the rear wheel
cylinders.  This condition is NOT normal and should be repaired for
safety.
Cheers,
John Franklin
San Rafael, CA
85 Spider, 86 Syncro GL, 87 505 Turbo S