Between the master cylinder and the wheels on many vehicles sits a small block called the combination valve. It quietly does several safety jobs at once. Here is what a brake combination valve is.
Three jobs in one
A combination valve typically combines a proportioning valve, a metering valve, and a pressure-differential (warning) switch in one body. Together they balance and monitor the hydraulic system - see brake proportioning valve - which is why it is called a combination valve.
Proportioning and metering
The proportioning function limits rear pressure under hard braking so the rears do not lock first. The metering function slightly delays the front discs so front and rear apply together on vehicles with front-disc/rear-drum setups, keeping braking balanced.
The warning switch
The pressure-differential switch senses if one circuit loses pressure (a leak) and turns on the red brake warning light. So a combination valve is part of what lights that warning when a hydraulic problem occurs.
When it matters
A stuck combination valve can throw off front-rear balance or trip the warning light; it is worth checking during a hydraulic diagnosis. Keep the whole system healthy with fresh fluid and quality pads and hardware.