This looks the same as the setup on a Jaguar E-type 4.2. Sticking brakes is a known problem with them, if you search the web for “Sticking Jaguar reaction valve” there’s lots of info available. As far as the Jaguar system is concerned, there’s a couple of different sources of the problem, but initial symptoms are similar. The little push rod in the end of the master cylinder can stick and fail to retract fully. This causes the reaction valve to open and servo to operate, even when the pedal isn’t pressed. The reaction valve and the pushrod are only held closed by a small spring under the reaction valve cover. This can corrode slightly and go weak over time. Dirt in the reaction valve could prevent it sealing properly, or a split diaphragm could also cause problems. The other area to give problems is the servo. On the Jags, at least, this has 2 independent hydraulic systems in it. These are separated by a shuttle piston that’s free to move along the servo bore to balance the pressure in the two systems. Pressure in the primary system is transferred to the secondary system through this shuttle. Corrosion and debris in the servo bore can cause this shuttle to stick, retaining some pressure in one or other of the systems. A final area to check is replacing all the vacuum hoses on the system. These can go soft and collapse, holding the vacuum on when it should be released. Be sure to use proper vacuum hose, heater hose is no good.