A good while back, I decided to rebuild my Girling servo and was unable to complete the task when I found a crack in one of the unavailable air valves. I replaced the Girling with a Lockheed unit and easily designed brackets to mount the unit onto the same studs holding the Girling. Over the next year I used the Lockheed unit and found it performed passably but provided lass boost then the Girling. Eventually I sourced new air valves and rebuilt the original Girling which has been in place and in use since.
I found the Lockheed produced less assist and found I preferred the added power of the Girling. However, the Lockheed did perform well and provided a good alternative.
A year back, friend was thinking of boosting the brakes in his BN6 and told about the PBRVH40 (
https://www.hydroboost.com.au/boostertypes.htm) by a Healey enthusiast out of Hong Kong. A smaller Lockheed look-a-like, the unit used by the Hong Kong Healey owner was purchased on eBay and supplied out of Australia. According to his recommendation, of the 2 single-line models available (VH40, VH44), the VH40 provides a higher brake boost with the VH44 used on disk brake equipped vehicles. Since I found the Lockheed to provide less boost than the Girling, I understand, the VH40 produces quite a bit more boost than the VH44 and details of both units can be found if you Google the unit..
Although I have no experience with these units, I though I would pass on their existence and maybe another forum member can provide additional information.
All the best,
Ray (64BJ8P1)