Lucas Girling drum brakes

I am having to do work on a 1971, F600 Lucas Girling brake system that I have not had good luck finding a source for service information to work on this brake system. I would like to find a source for service information on this brake system but I don't know where to look anymore. I know that this rig has 15X3 front drum brakes and (I think 15X5, not sure on the width of the shoes) rear drum brakes operating in a "single" hydraulic system in which the master cylinder on the firewall sends pressure to the hydro-vac unit, under the left rear corner of the cab, which then supplies the same boosted pressure to all 4 brakes by going thru a Tee fitting coming out of the hydro-vac. There is no proportioning valve on this truck.

Thanks in advance for any help!!