I’m having a strange brake issue that I can’t seem to find any information on. I had to replace the 2 lines that run from the front to the back. Ever since I’ve replaced the lines I can’t get the brakes to work. I’ve bled the brakes over and over again. I’ve replaced the master cylinder. I get the same result over and over. I get pressure while I’m bleeding, but when I start the truck all pressure goes away. The pedal goes straight to the floor. I don’t see any air bubbles coming out when I’m bleeding. Can anyone help?

 

Are you sure they are not swapped at the back at the Load Sense Valve?
Bleeding the whole thing starting at IIRC one back wheel and ending at the front??? is a unique process.

 

You didn't mention how you bled your brakes or IF you bled the load sensing proportioning valve. It sounds to me like that may be your issue or like ghill said, maybe you inadvertently swapped the lines.

 

So to be honest this is my first time messing with a brake system, and I’m not sure what or where the LSPV is. Would this be it, above the right rear wheel?

 

I don’t think that I accidentally swapped the lines. I flagged them so I wouldn’t. if they were swapped though would that give me this issue? Maybe I did accidentally swap them

 

Yes, that is your LSPV or Load Sensing Proportioning Valve, the arm attached to your rear axle senses when the rear of the vehicle squats so when you put a heavy load in the back it will send more fluid pressure to the rear brakes and when unloaded it reduces the pressure to help prevent rear wheel lockup.
There is a bleeder screw on the valve and you likely have air in it. I don't remember the proper method of bleeding this so check the service manual or maybe someone else will chime in with the steps.
Also, if the arm gets bent it can mess with your rear brake operation.
Since your pedal goes to the floor it's likely air in the lines.

 

Also, as a general rule, bleed from farthest wheel from master cylinder to closest wheel. I'm guessing the order here should be rear passenger, lspv, rear driver, front passenger, front driver. Someone can correct me if the lspv should be bled at a different stage.

 

According to the FSM, that's correct, except that the LPSV is LAST. So yeah, farthest from Master Cylinder to closest, and THEN the LPSV.

Good luck!
Pat☺

 

Ok, did you bleed the master cylinder before you installed it ? if your pedal is going to the floor you got air or internal bypassing on the master.