Hi Peter,
I am not at all familiar with a 411 master cylinder or any variations of it, however I have experienced the problem you described on two separate instances. ( not in a Bristol)
The first situation required overhauling the master cylinder, I suspected that the bleed valve in the cylinder was not operating correctly. It worked fine afterwards.
The second situation was the activating rod between the pedal and the master cylinder piston had a /lengthening/shortening adjustment by way of a threaded section into a coupling connecting to the pedal. This provided an amount of free play between the piston and the pedal ( much like the need for valve/tappet clearance) - but in this case not much, maybe 1/4 inch, just enough for the piston to return freely into its neutral position and not be restricted thus allowing the brake fluid to return without a resisting pressure, which if pressure was there would build up and eventually lock the brakes, as if you had your foot on the pedal.
The second situation was also very quickly fixed by way of loosening the bolts holding the master cylinder to the firewall because the adjusting coupling was very difficult to get to, at night in the middle of no-where.
Hope the above is of some help even though it was not in a Bristol but in a Chrysler/Mitsubishi.
Cheers
Pete
Ps. What type of accommodation do you want at Mildura :-)
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