I have to bleed the brakes on my 79 F350 4x4. The Haynes Manuel says the button/rod needs to be pushed in on the pressure diff. valve for trucks over 6800 GVW and pulled out for trucks under 6800 GVW. My truck says its 8550 GVW. The button or rod on my pressure differential valve pulls out and won't push in, just the backwards of what the book says. So what gives here, someone change the valve and put the wrong one on it or what. Or just pull the rod out and bleed them. Can those clips to hold the button open be bought anywhere ???

 

Just open or close it as you see fit. I haven't noticed a difference in my brakes if I did it or not.

Yes, there is a "tool" made that holds the valve rod out - a small needle nose vice grips and a wooden wedge....

But, actually, there is a tool made just for this occasion but I doubt you'll be able to find it anywhere. It's a small piece of spring steel bent over on itself with a notch in the middle, almost end to end that holds the rod out.

 

If the brake valve body is constructed of brass (Weatherhead brake valve), the pin is supposed to be pushed inwards. If the valve body is constructed out of cast iron (Kelsey-Hayes brake valve), the pin is pulled outward.

 

And as we've found out many times, the books are not always written by people who were given the full story from the people who put the parts together.
And sometimes even they didn't know which one went where and when!

So like they said, if yours pulls against spring pressure, pull it. If it's pushable, push it.
For comparison, my '79 F350 w/8550 GVWR has a cast-iron valve and it pulls out just like yours.

Paul

 

If you just pump the brakes slow it shouldn't bother the valve.

 

Just a thought... I made one of those tools to hold out the pin. I used the belt clip off the back of a tape measure. I cut a notch in the long side. worked great for me

 

Very clever! Good idea. I lucked out and a buddy have a pair of nice locking pliers with a notch in the jaws just the right size to hold, but not damage the shaft.
Pulled it out with one pair of pliers, clamped the locking ones on, and bled away.

If the delay valve (which is what that metering valve actually is for) is working properly, you should actually be able to see the little tab moving in and out as you apply and release the brake pedal.
It's just a simple spring loaded valve designed to delay the application of the front brakes until a tiny bit after the rear brakes have had a chance to start slowing things down.

Paul