Brake pedal hisses and goes to the floor

Good morning,

The hissing could be either the brake booster leaking vacuum if that is the noise you hear or it could be a brake line that is leaking fluid when you push the pedal down.

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

Do you have a puddle of fluid under the truck to indicate a leak?

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

Was this the original issue you replaced the master cylinder?

Roy

There was was no puddle of fluid under the tires. We were able to change the brake pads and bleed the brakes just fine. We even used the truck a few times but gradually we could feel the brake pad getting softer and going closer to the floor. We changed the master cylinder assuming that was the cause but the problem continues. Does this truck have a brake motor? There are two lines from the master cylinder that run into a cylinder. We haven't changed the brake booster it's hard to disconnect it from the pedal.

There is an ABS unit if it has ABS.

It sounds like there is air in the system.

This is the process for bleeding with ABS.

ANTILOCK BRAKE SYSTEM (ABS) BLEEDING PROCEDURE

NOTE:
^ The two person bleed procedure is required when installing a new electro-hydraulic control unit, or when air is suspected to be trapped in the valve body.
^ Do not run the scan tool Function Test after combination valve or tube adapter replacement.
^ Do not drive the vehicle until brake pedal feel is firm.
^ Do not reuse brake fluid used during bleeding.
^ Vacuum, pressure or gravity bleeding may be used for base brake bleeding only.

1. Raise and support the vehicle to gain access to the system bleed screws.
2. Begin by bleeding the system at the right rear wheel.
3. Install clear hose on the bleed screw. Immerse the opposite end of the hose into a container partially filled with clean DOT 3 brake fluid.
4. Open the bleed screw 1/2 to 1 full turn.
5. Slowly depress the brake pedal until it reaches its full travel and hold until the bleed screw has been tightened. Release the brake pedal and wait 10-15 seconds for the master cylinder pistons to turn to the home position. Repeat until clean, air free brake fluid is present at the wheel bleed screw. This procedure may use more than a pint of fluid per wheel. Check the master cylinder fluid level every 4 to 6 strokes of the brake pedal to avoid running the system dry. Repeat steps 3 though 5 on the left rear, then right front, then left front.
6. Use the scan tool to run Function Test four times consecutively while applying the brake pedal firmly. Remove foot from the brake pedal between each test.
7. Rebleed all four wheels using steps 3 through 5 to remove the remaining air from the brake system.
8. Evaluate the brake pedal feel before attempting to drive the vehicle and re-bleed as many times as necessary to obtain appropriate pedal feel.