From your post I take it that you have the workshop manual ? ( if not, I have the 44 page MD7A manual in pdf format, if you need)
For Info, the CAV pump is now the Lucas CAV.
From the brief description it looks like a slow leak somewhere. Can you see into
the primary filter if so, can you see a small bubble trail when the engine is running ??
In the manual I have, para 75a 0n page 28 gives bleeding order. On page 33 there is whole section dealing with bleeding. Maybe the best bet is to replace all washers - 'O' rings, copper compression washers etc.. and the re-bleed the whole system.
Parts Link HERE