Jegs proportioning valve concerns

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delta_p

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Here's what I am having trouble with. The figure shows how i am seeing the combination valve with brake pedal pressure being applied in normal mode, leak to rear, and leak to front. In leak to rear I see no blockage really. In leak to front, I see blockage to the front, I see the rear opening up a bypass around the proportion valve (probably to allow full pressure without reduced clamping of pressure and to allow some braking without pushing the metering valve much). Like the meter valve is being held closed. And normal mode is obvious to me. I am coming around though i promise :).

Except in normal mode as I believe you stated the metering pin stays as shown frm 0-25 psi on the front, then shuttles to close the meter valve from 25 - 100 psi, then the pin and valve shuttle to wide open the meter from 100 psi up.

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Schurkey

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I "blew up" that diagram to 300% on my screen, but the resolution is just not there.

Yes, I see what you mean about fluid flow seemingly bypassing the proportioning valve during a front-circuit leak. Apparently, they want absolute maximum pressure on the rear wheels. I guess that makes sense, there'd be less weight transfer from rear to front simply because the front wheels aren't doing much braking. Therefore the rear wheels would be able to accept more braking pressure since the front brakes are "along for the ride" instead of fully-functional.

It seems the safety piston is mechanically cramming the holdoff valve to a low-flow/low pressure condition. I don't know why--the valve should already be in the low-flow position since there's low pressure in the leaking/failed front system. Total mystery to me.

What combo valve is this? GM? Ford? Chrysler? What application, do you know?
 

Chris253

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I watched a YouTube video where the guy just took the small lines off the proportioning valve and master cylinder to see into the valve and just pushed the piston back to center. Said that it solved his problem with not being able to bleed the brakes.

Well, I finished installing the proportioning valve on my truck and it sure did do a lot of good but I believe that I still need to bleed them a little better and reset the valve, also noticed the parking brake was frozen so I disconnected it just for now, but the drums are adjusted tightly.

One other thing that I noticed is that the brake booster that I just replaced makes a whoosh sound at the top of the pedal, after it does its whoosh the brakes will catch.

I also noticed that when I hit the gas pedal hard and push the brakes real quick while the rpms are higher it will gain more pressure at the top where the whoosh is. I'm starting to think it's a vacuum issue? Correct me if I'm wrong please thanks
 

Chris253

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One other thing I should add to my suspicion about the brake booster is that it's not stiff at all to push the pedal, it's very loose and doesn't return as quickly as it should during normal driving but does return and gets stiffer when the rpms are higher

Also the booster is only about 3 weeks old.
 

delta_p

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There's a valve in the booster that opens when you step on pedal and allows atmosphere pressure into the pressure chamber side of the diaphragm. It was at vacuum before the valve opened. The other side of the diaphragm is always at vacuum. The atmosphere on one side and the vacuum on the other (along with pedal force) is what moves the booster pushrod to the master cylinder. Maybe the sound is the chamber filling with atmosphere. Not sure why it would seem like a delay in charging (catching after the woosh) unless the valve isn't operating smoothly? There is supposed to be something in there that dampens out the sound from the valve opening.
 
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