To be clearer, the DOT 5 total synthtic is what Girling are advising not to be used in Step cylinders. Though there has been the general view that DOT 5 is unsutable for older break systems for a long time. Rather like 20/50 oil, DOT 4 is the improved version of DOT 3 suitable for sytems originally designed for DOT 3. But DOT 5 is like a synthtic 5/40 oil, just not compatable. I have always used DOT 4 on my Dodges and Land Rover Series with never a problem, so if manufactuer Girling recommend that for the system, I'll use it. Back to the old adage a Bugger if it don't go, a Disaster if it won't stop!
Same with rebuilding, I'm not happy to do so as I don't have the confidence in my ability to do it properly, so I have bought new.