Brake pedal goes to the floor after replacing brake pads

HI,

If the brake pedal is going to the floor and there are no leaks and you didn't open the system allowing air into the system, chances are the master cylinder was damaged. I have actually seen the plungers damaged in the master cylinder from fluid being forced the wrong direction.

As far as pushing the caliper pistons back, you had to do it. Years ago I was taught to open the bleeder on the caliper when depressing the piston to prevent damage such as this and to remove dirt in the caliper.

If you have no brake pedal and there are no leaks or air in the system, replace the MC.

Here are a few links to help:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Here are the directions specific to your vehicle. The attached pic correlates with the directions.

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2008 Nissan-Datsun Truck Pathfinder 2WD V6-4.0L (VQ40DE)
Removal and Replacement
Vehicle Brakes and Traction Control Hydraulic System Brake Master Cylinder Service and Repair Removal and Replacement
REMOVAL AND REPLACEMENT
Removal and Installation

BR-20 COMPONENTS

Pic 1

CAUTION: Be careful not to splash brake fluid on painted areas; it may cause paint damage. If brake fluid is splashed on painted areas, wash it away with water immediately.

REMOVAL
1. Drain brake fluid.
2. Disconnect harness connectors for fluid level sensor and pressure sensor.
3. Disconnect brake tube from master cylinder assembly.
4. Remove master cylinder assembly nuts and master cylinder assembly.

INSTALLATION
Installation is in the reverse order of removal.
Refill brake fluid and bleed air.

CAUTION:
Refill using recommended brake fluid.
Do not reuse drained brake fluid.

Adjust brake pedal. Refer to BR-7, "Brake Pedal Inspection and Adjustment". See: Brake Pedal Assy > Initial Inspection and Diagnostic Overview > Inspection and Adjustment

Let me know if this helps.

Joe