I have replaced the brake lines, master cylinder, the vacuum hose from the carburetor to the vacuum booster. When the truck is off, I have all the brakes in the world. When I start the truck, the pedal drops to the floor, and takes 2-3 seconds to return. If you know the answer, solution, or where I can find the answer, please let me know. Thank you.
Its in the vacuum booster. Diafram ,maybe but their is a kit to rebuild it
You're standing on it, evaluate it as if you were braking normally, inspect the mounting of the pedal linkage, the mounting of the booster and the master, run vacuum tests for supply and volume and control. Vacuum volume requires a manometer, or comparison testing. Your symptoms suggest the reverse is true, too much boost. You may have the boost assist occurring to make the problem Happen, and show itself, you need a tech to inspect while you make it act up. A mechanical issue on the apply side for the booster, or internal in the apply side can exist. almost a 40 year old truck, that means any weird set of circumstances or inproper assembly, or damage can exist. A previous collision, or bent part issue from hard application, or a failing firewall bulkhead. This happens to much tougher trucks than yours, so dont be surprised if you find something.
I own a shop, and have my share of dealings with brakes. First of all lets think about this: The brakes are mechanical, and work just fine until you turn the engine on, so this indicates the master cylinder is fine, and the problem somehow lies within the brake booster. Don't think it is bad just yet! The vacuum line that supplies the booster may be the problem. There is a check valve in the line where it plugs into the booster. If its bad, then the booster will rob all the vacuum when you start the engine, and the pedal comes back once it runs some, so I think this valve may be bad. Take it out, and see if it hold pressure one way but not the other. If so, then its working right. With the engine running, unplug the hose, and it should kill the engine, or very near cause it to stop running. There may not be enough vacuum on the line, and it could be plugged up under the carburetor with carbon. You should be able to run a clothes hanger into it to open it up. I've seen some that had to be drilled out because the stuff was so hard, and had accumulated there for years. Either way. make sure you have a good supply. Make sure you have a good hose that doesn't leak, and the valve that plugs into the booster where the hose plugs onto should be able to suck air out of the booster, but not suck air the other way. In other words it lets air flow one way, but not the other. Since this is vacuum it should flow out of the booster, and not into the booster because you are dealing with vacuum, and not air pressure. If you determine the supply is good, and the valve is good, then if it doesn't work replace the booster. It must be one like what comes on it, or you could have a problem with the length on the rod for the master cylinder. You could even have the wrong booster on it. Glad to help out, Good luck!!!
