The brake pedal isn't really supposed to go down very far in the first place. The amount that the pedal is allowed to move is expressed as "pedal reserve". 75% is the absolute minimum. Think about how little throw there was when your brakes were working properly - basically less than a 1 inch depression was probably required to stop the truck.
Check Brake Pedal Travel (Non-Antilock Brake Systems Only)
- Fully depress the brake pedal and check for the following:
- The pedal should have a 75% reserve.
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- The pedal should be firm and not leak down.
Prior to replacing a master-cylinder for poor pedal travel or feel verify the following:
- There were no problems with any of the related systems.
- The entire brake system has been bled.
- Rear brakes are properly adjusted.
There are no real return springs, so if it goes way down, it is staying down.
Either the tech who bled the brakes after the line replacement didn't do it correctly or if the rears were worked on they may need to be adjusted out.
One other possibility comes to mind - there is another leaking section of hard brake line or maybe a bulging brake hose.
