Power assisted brakes have been with us now for many years. Vacuum is taken from the intake manifold and used to boost the hydraulic pressure applied by the driver via a large rubber diaphragm in the brake booster. In most manufacturers applications, manifold vacuum is taken from the large plenum chamber in the intake manifold between the engine and the butterfly or butterflies in the carburetors or injectors, this is an ideal arrangement, as long as the motor is running there is usually up to 23 inches (HG) of vacuum to operate the brakes. All the cylinders of the engine feed from this chamber enabling the system to re generate vacuum quickly.
Ferrari, over the years beginning with the 250, but continuing on through the evolution of the 2 cam motor right up to the 365, has utilized only vacuum from the rear carburetor of the engine to generate vacuum for power assist to the braking system. For example, most of the 2 cam 3 carb engines use only the rear carb manifold to generate vacuum, this manifold serves the rear 2 cylinders on each side (Cyl # 5-6-7-8 ) The amount of vacuum generated is sufficient to stop the car under normal driving conditions provided one does not pump the pedal excessively. On idle when the engine speed is insufficient to regenerate vacuum quickly, three or four fast applications of the brake will usually result in a very hard pedal with very little stopping ability. Basically, once all the boost vacuum is gone, all that is left is your leg strength.
This was not a problem, rather something one needed to get used to. When the 6 carb engines were introduced, particularly the 275 4 cam which had rather radical valve timing, (any increase in camshaft duration results in a decrease in manifold vacuum) on these cars the rear carb manifold only draws vacuum from 2 cylinders so there is almost no reserve. In addition, due to the small displacement of the cylinders ( 2 X 275 = 550 cc) they are unable to regenerate vacuum quickly. On idle, two brake applications can exhaust the vacuum supply on a 275 4 cam. This problem exists to some degree on all pre-fuel injected 308's and Dino's, both of which pull vacuum only from the left rear intake port. Some cars, 365GTC/4, Daytona, 512BB, 400 series were fitted with a small vacuum pump driven usually directly off the end of one camshaft to assist vacuum at low engine speed brake applications.
Adapting one of these pumps is no easy matter, impossible on some cars, but there are a couple of solutions which will work very well on all 2 cam, and 275 4 cam motors. One requires removal of all the carbs and intake manifolds, the manifolds need to be jigged (accurately set up) so that a .500" hole can be bored through each of them, passing through the plenum chamber under the carb. Aluminum tubes are accurately machined to join all the plenums together so that all 12 cylinders are now supplying vacuum to the power brake or servo unit. The tubes must be machined to the correct diameter on the ends and the correct length, "O" rings are fitted to each end to prevent leaks. No vacuum leak can be tolerated anywhere in the intake system. Any amount of leakage into the intake system whether by leaks or improperly closing or worn throttle shafts, particularly on 6 carb motors will result in the inability to get idle speed under 1500 RPM, that is simply because with all those carbs, each one does not need to "bleed" much air past the butterflies to give the motor enough air to want to idle at 1500 RPM or above.
The other solution is to mount a reservoir somewhere in the car to hold additional vacuum. This requires the use of a one way check valve (from a 308) so that the reservoir will always hold vacuum and not let it escape back into the intake manifold when the motor is turned off. Old fire extinguisher bottles are sometimes useful for this purpose