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Old 04-19-2012, 11:44 PM   #1
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Join Date: Nov 2011
Location: Kirkland, WA
Posts: 343
Stubborn Brake Bleeding Best Practices

Below is a compilation gained from many members for a stubborn problem I had getting my new disc brake conversion to bleed. Their information solved a very frustrating problem and can be useful for any bleeding process.

Special thanks to CDowns, Kevin LFD, Inc. AR15Defense, 69 halfton, CCP to name a few and others for sharing their knowledge. Most of the below is direct quotes from their posts. There may be another “best practices” thread, or there may be better information available. If so please add a reply. You or someone you know might find it useful someday.

Brake Diagnostics:

1. Check to make sure you have proper hook up from Master Cylinder. Is the front of the Combination Valve (Proportioning Valve) going to the Front brakes and the rear going to the Rears? YES/NO

2. Make sure bleeders are on top of calipers YES/NO [INDENT]a. You want the trapped air to rise up to the bleeder, it will not get forced to bottom with pressure.[INDENT]b. GM calipers can be swapped side for side which at times puts the bleeder on the bottom and traps air in the top of the caliper.

3. Make sure there are no “loops” in the brake lines or high spots above the bleeders that can trap air such as the hoses! YES/NO (This was my problem. When I attached a drain hose to the bleeder and ran it into a paper cup with brake fluid in it, the hose bubbled like a fish tank air pump!)

4. Crack the lines out of the MC, does fluid squirt or gush out when you push the pedal? YES/NO, [INDENT]a. If NO then something is haywire in the MC or linkage. [INDENT]b. If YES then move to the next joint at the proportioning valve #6.

5. Does the pedal move the MC rod far enough to move fluid? YES/NO
a if NO then be prepared to adjust your linkage (sometimes this means cutting the push rod about a 1”).

6. Crack the lines at the Proportioning Valve outlets. Is there a gush of fluid when you push the pedal? YES/NO
a. If NO then the proportioning valve may need to be centered (this usually only affects the front or back not both. It is a safety device used if loss of fluid occurs. To “center the valve”, either jam the brakes really hard or push in the pin that is covered under a rubber protector. With the pin depressed it will allow fluid to go to both front and back systems.
b. If YES move on down the lines checking for leaks at any connection point or for ruptures in the brake tubing to the Bleeders.
i. If your proportioning valve is lower than the calipers (usually the case with lowered rigs) you will need check valves in the lines to keep pressure to the calipers.
7. Check for fluid at the Bleeders, YES/NO
a. if NO then you have either a blockage such as a pinched brake line, or a leak in the lines.
b. if YES then it is very likely there is still air trapped in the system, my bet is it is in the hoses as described below or a problem in the caliper.
8. Then at the calipers. YES/NO

If YES then you have fluid at the calipers and they try to squeeze and/or don’t leak so you very likely do not have a malfunctioning part or a blockage of some kind. However, you will need to bleed the brakes again but this time use a “C – clamp” to push the pin in on the combination valve while bleeding the brakes for proper bleeding operation. If not then the metering proportion valve will divert your fluid and you will mot solve your problem. Air in the system occurs usually after repairs or damage. Also, air can be sucked into the system at anyplace a leak can occur. Checks for leaks, even minute ones especially where ever there is a fitting. If no visible leaks then Bleed this way:
a. There is a pin at the front of the combination valve, sometimes it’s covered by a rubber bulb. GM has a special too for holding it in while bleeding brakes but a “C – clamp” will work or have someone hold it in. It must be held in position.
. The front bowl is for the front brakes for front disc brakes. Fill your reservoir bowls with new/clean fluid.
ii. Begin bleeding at rear drivers side because it is the longest brake line run making it the furthest away by length of line distance, then the rear passenger side, then the passenger side front, then the driver side front. Do not allow the fluid reservoir bowls on the master cylinder to become empty. Frequently refill the reservoir bowls with new/clean fluid.
b. Because disc brake conversions include new front and/or rear rubber hose lines you may still have air in them after struggling with a long and difficult bleeding process. The master cylinder simply cannot push enough volume of fluid to get the volume of air out of the hoses.
i. Wire the hoses down so that they are lower than the bleeder's. This will force the air up to the bleeder. I pulled my hoses down below the bleeder or have someone hold them down.
ii. Bleeding process simplified. 1) Have a helper depress and hold peddle down, DO NOT PUMP, you could possibly suck air back into the bleeder. 2) Attach a tubing hose onto the bleeder fitting and run the tubing into a clear capture bottle or jar containing several ounces of brake fluid. 3) Submerge the other end of the tubing hose into the clear fluid, this will help you to see bubbles of air being forced out of your system. 4) With the brake peddle depressed – slowly release the bleeder valve approx 1/4 turn – watch for air bubbles escaping from the tubing into the capture jar fluid. 5) When brake peddle travels to floor and then stops have the helper continue holding the peddle down until the bleeder valve is closed. 6) Once the bleeder valve is closed the have the helper depress and hold the peddle down again. 7) Continue bleeding the line until no bubbles appear or until the capture bottle is full. I personally like to get 4 more times of no bubbles before I am satisfied. Refill reservoir bowl and repeat the process for the other side or other wheels. 8) If you are having difficulty getting all the air out of the system or you are not obtaining a firm pedal, try these steps to assist in the bleeding process and track down problem areas on the vehicle. A spongy pedal is usually caused by remaining air in the system. While bleeding, raise the end of the vehicle that is being bled, this may help air trapped in the system to rise towards the bleeder screw. While bleeding, lightly tap the caliper with a hammer. This can dislodge air bubbles that cling to the walls of the caliper reservoir.
"Speed bleeders" allow one person to open the valve and pump, they have a built in check valve.

"Centering" a proportioning valve = stomp on the brake pedal as hard as you can while transmission is in “Park”. The brake light should go out and the P valve is re-centered! Also the P-valve can be centered with a small pick with the switch out.

"Combination Valve" includes a Metering Valve, a Proportion valve and the Brake Light Switch. The pin that sticks out or up is part of the Metering Valve and has nothing to do with the Brake Light Switch.

Note: If the Metering Valve (or Hold-off Valve as it is also called) is bad, the vehicle will nose-dive at low speeds with light brake pressure. It prevents pressure to the front disc brakes until the rear brakes reach about 40 psi. If one side (front or rear) fails, the pressure from the good side pushes the Switch piston to the other side, which should close off that line out of the combination valve. That way if the problem is a rusted through or broken line or leaking wheel cylinder, you are not pumping all the fluid out onto the ground.
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