G’day
Just read your query and I might have the solution if the same has happened to you as it did to me when I first bought my E model 15 years ago, I found the brakes poor and would go to the floor, but if I pumped them fast the brake come up hard and responsive, after a bit of looking around, in a previous bit of work someone had left out the wave washers on the master cylinders ( allowing a little pressure on the cup and not allowing a slow bypass of pressure when the brakes were applied, fast reaction would hold these in place, slow action would allow a slow bypass) found a couple of washers and they’ve been perfect ever since
I’ve always pressure bleed my brakes from the calliper with an oil can. Never any problems
Hope this is helpful, let me know
Cheers
Darren