What does this statement in the shop manual mean about bleeding brakes?

[davepl]

According to the 72 manual, it says:

"NOTE : The combination valve, located near the master cylinder, must be held in the open position while bleeding. This can be accomplished by
installing Tool J-23709 with the open slot under the mounting bolt and pushing in on the pin in the end of the valve. Be sure to re-
torque the mounting bolt after removing the tool"

So what does it mean to "hold open" the combination valve? Why is it required?

Thanks,
Dave

PS: Please don't say it's not required unless you know why they thought it was required...

[TBONE1964]

Originally Posted by davepl

According to the 72 manual, it says:

"NOTE : The combination valve, located near the master cylinder, must be held in the open position while bleeding. This can be accomplished by
installing Tool J-23709 with the open slot under the mounting bolt and pushing in on the pin in the end of the valve. Be sure to re-
torque the mounting bolt after removing the tool"

So what does it mean to "hold open" the combination valve? Why is it required?

Thanks,
Dave

PS: Please don't say it's not required unless you know why they thought it was required...

Sometimes it is not required but when it is required, it pushes the valve inside the proportioning valve back to center. If the valve is shifted off center it usually will only allow brake fluid flow to the front brakes and nothing to the rear. When the valve is centered, it allows brake fluid flow to flow equally to the front and rear brakes. The tool helps push and hold the valve into the centered position while the brakes are being bleed. Once bled the valve will stay centered.

Another use for the valve is if there is a loss of fluid pressure, front or rear, it will shift the valve and trigger the brake light on the dash. That is one of the purposes of the valve.


Hope this helps

[WIDESIDE72]

I was able to use a c clamp to compress the button. In the past I have seen a flat strap with two mounting holes matching the prop valve with an angle bent on the end to compress it.

[cdowns]

i too use a cclamp to hold the pion in// it allows you to bleed the rear brakes without the need to change lines or rubber hoses or master cylinder that the guys that dont read the service manual do

[davepl]

Does it matter that my brake light switch isn't on (not grounded)? Or does checking it before/after not prove anything, as it could be sliding around?

Is there a diagram or picture? I'm trying to imagine what I could do with a C-Clamp and my prop valve. Do I remove the sensor on top to access it? Does opening that sensor port allow air into the system?

Need more info! Thanks...

[TBONE1964]

Originally Posted by davepl

Does it matter that my brake light switch isn't on (not grounded)? Or does checking it before/after not prove anything, as it could be sliding around?

Is there a diagram or picture? I'm trying to imagine what I could do with a C-Clamp and my prop valve. Do I remove the sensor on top to access it? Does opening that sensor port allow air into the system?

Need more info! Thanks...

Here is a cut away view. The circle shows the button that you would push. You can use a C-Clamp across the valve to push the button in. Or buy the special tool. The connector in the middle of the valve is for your brake light. IF you look at the valve inside, you can see that if it shifts one direction or the other, it is going to ground the connector and that will illuminate the brake light. You do not need to remove anything. If your system is working correctly, if the valve is shifted, your brake light should be on.

To test the brake light function, remove the wire from the valve connector and ground it. If the light comes on the dash when you ground the wire, the light should come on when your valve is shifted. If the light is capable of working but does not come on when it is pluged into the valve, then my guess would be that your valve is OK and not shifted.

[Fingle]

Awesome explanation, thank you!

[davepl]

My brake light is functional (comes on when the wire is grounded) and not on (no ground present) at the valve connector, so I think I'm ok.

So does this "button" prevent the pistons from de-centering? In some designs you have to remove the center switch and poke it around with an awl, so that'd be an improvement.