brake booster question

[Mike_The_Grad]

Hey Tom, any update to your situation with the brakes?

[El Dorado Jim]

Originally Posted by Mike_The_Grad

"Brake pressure from the front and rear port are the same as long as there is no failure in the MC or any other part of the brake system."
True statement.

each piston displaces brake fluid proportionately to their input.
Also a true statement.(layman's terms)


The Metering Valve HOLDS OFF fluid from the front brakes until line pressure reach's 40-60 lbs, depending on application. This holds off application of the front brakes until the rear brakes can begin to work and prevents nose-diving of the front end.
True statement.

some biasing takes place to prevent premature application of the front brakes and over application of the rear brakes.
Also a true statement.(layman's terms)

The warning Switch Piston does not move if the pressure on both inlet ports is the same.
True statement.


>>You must keep the pin depressed while bleeding the brake system or your brake system warning light will turn on inside the dash cluster<<

I apologize for this statement, as it needs some clarification.

"The pin" that is "depressed" in actuality is a preventative precaution. And is not so much "depressing" it, but rather preventing the "warning piston" from becoming un-centered during the brake system bleeding process.
If air is present between the outlets of the MC and the inlet ports of the "combination valve" this can result in an un-equal pressure variance between the front and rear brake systems.
It is possible that no air exists in this section,which would allow you to successfully bleed the brake system without an un-equal pressure variance between the front and rear brake systems.

But, how likely is that?

The factory service manual instructs the technician to take this precaution while bleeding the brake system, so as to avoid the added time of re-centering the warning piston when it could have been prevented in the first place.

I'm confused on how to keep that pin depressed or how to keep the piston from moving so the light wont come on?

[Mike_The_Grad]

In the factory service manual, there is an actual tool number that they suggest. I can't tell you off the top of my head what it is, but the "design" of it is basically a piece of steel of gauge thickness,about an 1" or so wide and about 4" long that has the last inch or so bent into an "L" shape with a "U" groove in the end of the long side of the tool, this end is slid under the front mounting bolt of the combination valve. When installed the leg of the "L" is aligned with the "warning piston" and acts as a "stop". I made my own out of a piece of sturdy sheet metal that I cut with my "tin-snips" and used my bench vice as a brake press. I then used a small "C-Clamp" to clamp it to the combination valve, making my version a little shorter than the one that mounts under the front mounting bolt. Worked fine for me.

[RichardJ]

>>"The pin" that is "depressed" in actuality is a preventative precaution. And is not so much "depressing" it, but rather preventing the "warning piston" from becoming un-centered during the brake system bleeding process.<<

Not at all how it works.

The Metering Valve opens when the pressure exceeds about 50 psi. When there is air in the lines and you are trying to bleed the air out, THERE IS NOT ENOUGH PRESSURE to open the Metering Valve and it is very difficult to bleed the air out of the lines.
If the pressure will not open the valve, THEN YOU MUST MANUALLY OPEN the valve.

There are TWO TYPES.
Some Valve Pins must be pulled OUT to open them and some Valve Pins must be pushed IN to open them.

The two types of tools for the two types are shown below.

There is no physical connection of any kind between the switch piston and the Metering Valve.

[Mike_The_Grad]

RichardJ everything you are stating is True. Except for your reference to one of my earlier posts. Nowhere did I mention "Metering Valve" or that there was "any kind of connection" between the "Warning Piston" And "Metering Valve" physically nor figuratively. However, there is a relationship between the "Warning Piston" and the "Proportioning Valve". There is NOT a "physical connection of any kind" between the "Warning Piston" and the "Proportioning Valve" UNDER NORMAL OPERATION of the "Combination Valve", i.e. driving and braking. We all know liquids cannot be compressed( in the normal realm of things), air however can and does compress under pressure. If air were introduced into the "Secondary Piston Bore" and the brakes were applied, the force exerted by the "Secondary Piston" on the Hydraulic Brake Fluid would begin to compress the air, normally this void would be occupied by Brake Fluid and the "Proportioning Valve" would begin to open at the correct pressure, the presence of air in this chamber, however, allows the "Proportioning Valve" to operate in a way that is not intended. Resulting in a "Physical Connection Between the Warning Piston and the Proportioning Valve". With the electrical circuit complete for the "Brake System Disproportioning Warning Light", we get that bright red "BRAKE" indicator light that we all love seeing on our gauge cluster after we "successfully and completely evacuated all air from the Brake system...."

[tommys72]

no update yet mike. Actually haven't driven the truck much since then. And also didmt know you had asked either. I havemt been on the site alot lately. Been super busy.