I have had exactly the same brake servo problem this weekend, despite fitting the rose joint and a Landrover return spring. But I did find how to solve the problem...............
I've just rebuilt my Cabrio having had it painted. I wanted to take it on a 400 mile round trip to the Tatton Hall Classic car show, so wanted to make sure the electric fan worked. I parked it on the drive and left it idleing to build up engine temperature. When I came back to it I discovered the brake lights were on. These servos definitely suck them selves on!
I still went on the trip, and managed without problem - the solution was to pull the brake pedal back up with my toe every time I used the brakes - it did not make for a relaxing drive, but it passed off without sticking hot brakes. I now know for certain, with my servo at least, the issue is due to the inlet valve not seating. In my case I know a stronger return spring, or one mounted further down the pedal to gain more leverage is the solution.
Try this with yours - brake, and immediately pull the pedal back up with your toe ( and make sure your brake light switch does not prevent the pedal returning fully) - in my car there is definite movement to be felt. We drove non stop for 190 miles home without the brakes sticking on, so I know it worked for me.