Nick,
Sounds to me like the servo may be the culprit and the fluid is leaking into the shell of the unit, if it was being sucked through into the engine you would see white smoke from the exhaust, I had servos on my 410 and 411 rebuilt as they were leaking internally, I had this done by past parts or Power Track Brakes (it was some years ago!)
This link while not specifically relating to the a Bristol may be of help or interest.
Geoff.
https://www.v8register.net/subpages/V8NOTE228.htm