Thanks for all the advice, in particular that from "old and brittle" Andrew, because it looks like old and brittle is the problem.
I replaced most of the vacuum hoses and fittings 2yrs ago, but one presented a problem: a plastic part connected via a short hose to the front of the intake manifold. The part supplies vacuum to both the distributor and the power brake. This part has seriously deteriorated into a crumbly substance, and the brass tube onto which the hose that goes to the distributor attaches has fallen out.
The problem is that this part is inserted into a formed plastic pipe that goes to the power brake, and it appears that the insertion is intended to be permanent. So, even if the plastic part is still available, what about the formed pipe?
I'm thinking I should get some plastic 'rebuilder' and try to repair the plastic part.
The short hose from the plastic part to the manifold is also deteriorated, but that can almost certainly be replaced.
rgds, nf