To "pump the brakes" means to slow down, take a step back, or pause before moving forward with something. It suggests exercising caution or moderation instead of rushing into a situation. Full definition
When you're braking and the car slips on snow, ice, gravel, or rain, ABS pumps the brakes on and off up to 15 times a second. (extremetech.com)
In an emergency stop, you just plant your foot as hard as you can on the brake pedal, and the ABS pumps the brakes and keeps the wheels from locking up. (articles.orlandosentinel.com)
ABS rapidly pumps the brakes during hard braking to help prevent your wheels from locking, helping you maintain steering control so you can better avoid obstacles. (nissanusa.com)