1998 Mitsubishi Montero how to test brake pump

Hello gio6191993

Welcome to 2carpros.

I could not find brake pump, do you mean master cylinder, power booster,
Can you explain what problem you have with your brake right now.
Thanks

Thank you for giving me more information

Check the brake fluid level, if the level is low, you might have a leak causing air to enter the system.
Now, if the brake fluid level is good and pedal sinks to floor when brakes are applied, you have a bypass in the master cylinder primary piston cup.
When the master cylinder's pistons and/or bore become worn, the pistons won't hold pressure and the brake pedal will gradually sink when the brakes are applied.
Here is what you need to do.
Open the fluid reservoir filer cap, while a helper is applying the brakes, look for strong waves (brake fluid movement in the reservoir)the bypassing on the primary piston is occurring.
If the master cylinder is worn, however, then replacement is recommended.

If the brake fluid is unchanged, and the pedal is sinking to the floor, the only possibility is the bypass in the master cylinder or depending how you describe the problem, there may be air in the system, but I doubt that one because you never have to bleed the brake, right?
Have the master cylinder checked and come with result.