Brake Proportioning Valve Question

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Sep 6, 2011 | 09:22 PM

JohnBOV43159

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From: Riverside, CA

Brake Proportioning Valve Question

Just put new shoes, pads, rotors, drums, wheel cylinders, calipers, master cylinder, power booster and braided steel hoses on my 1977 F150 4x4. I am ready to bleed the system and my question is:

Should I bleed the rear lines first and then pull the release pin on the proportioning valve or should I pull the pin on the valve before I start bleeding the rear brakes? I am guessing that I should put the vice-grips on the proportioning valve pin before I start, but does it matter when?

Thanks - John

Sep 6, 2011 | 09:25 PM

wyckedcombo17

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Yes, same procedures as normal bleeding. RR, LR, (pull the pin) RF, LF. When you get to the fronts is when you need to to pull the pin, the proportioning valve only controls pressure between the front two brakes, not F/R.

Sep 6, 2011 | 09:32 PM

ford390gashog

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The above info is about 50% correct the proportioning valve controls front to rear bias NOT front two brakes. It keeps the system at a 70/30%.

Sep 6, 2011 | 10:05 PM

bamaf150

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And someone here that sounded like they knew said the other day in another thread that the front an rear brakes are two totally independent systems, the valve only lets you know when there has been a loss of pressure in one of the systems by activating the brake warning light.......idk.

Sep 6, 2011 | 10:09 PM

wyckedcombo17

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ford390/OP I'm sorry, I was under the impression that that was why you only needed to pull the pin when you do the fronts. According my manual it looks like it only controls L/R pressure

Sep 6, 2011 | 10:40 PM

ford390gashog

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Quote:

Originally Posted by bamaf150

Kinda true! Before modern master cyls the front and rear brakes were feed by a sinlge chamber master cylinder and if anything happened you lost all brakes. Then fed standards went to mandatory dual chamber units separating the front and rear brake systems in case of a fault. The way it works is if you blow a brake line the proportioning valve should sense the high flow and trip shutting off flow to the affected system either front or rear. Its not a perfect world and don't bet on it to work! I know first hand that even with separate systems you still end up in a white knuckle situation rather fast! This really hold true if the master cylinder is old and starts bypassing fluid internally and then the prop valve has no chance of working.

Sep 7, 2011 | 09:44 AM

JohnBOV43159

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Thanks very much guys. My concerns were 1.) Is it better to bleed the RR then LR before pulling the pin while there is some isolation from the front system and then 2.) If after finishing the rear and THEN pulling the pin could some air from the front system seep back to the rear that was already bled?

After some thought I think I'll just pull the pin on the proportioning valve before I start the RR, I should be safe then.