A brake booster provides additional boost capacity without unduly increasing booster size and weight by providing a vacuum pump connected so as to add its additional vacuum capacity to that of the normal vacuum source when activated in response to sensed approaching run-out. The additional vacuum is provided by connecting the vacuum pump in a second conduit between the vacuum source and the booster vacuum chamber in parallel with a first conduit connecting the vacuum source directly with the booster vacuum chamber, with check valve means in the first and second conduits preventing substantial vacuum loss from the vacuum chamber. Approaching run-out is signaled by a sensor responsive to a vacuum level in the working chamber; and control means are responsive to a predetermined low value of the sensed vacuum level in the working chamber to activate the vacuum pump and thus increase the vacuum level in the vacuum chamber over that produced by the vacuum source, whereby a greater pressure differential is available across the booster diaphragm before run-out is encountered. An optional sensor responsive to vacuum level in the vacuum chamber enables the vacuum pump to be additionally used for backup of the vacuum source while still providing the functionality of the invention in most cases. The venting of the vacuum pump to the vacuum source reduces released noise which might otherwise require bulky noise filter apparatus.