I too had that problem with my '84 Westy and I even bought a new Master
cylinder.
However, I didnt fit it because I solved my problem by bleeding the master
cylinder by loosening the pipe fittings coming out of the body of the master
cylinder itself.
I can only suppose that due to the unusual position of it in this vehicle, it
is prone to pockets of air.
I had just replaced the rear slave cylinders when the problem appeared.
It may work for you too
Steve