Did I damage the proportioning valve?

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ericg75

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While waiting for engine parts I decided to bleed the brakes on the truck. All new calipers, pads, rotors up front and rebuilt drums in the rear. Also all the lines are new. Rears bled great with no problems with my son working the pedal and me underneath. Went to do the passenger side front and got no fluid. I cracked the line coming from the proportioning valve and it was dry. Tried blowing it out with compressed air as I figured maybe dirt was blocking the hole and nothing. I then took a needle and a mirror and pushed it up into the hole and found a lot of resistance. It finally punctured something (???) and the fluid started dripping. I reattached the line and was able to get fluid at the caliper. I would not have thought a piece of debris would cause that much resistance hence my question, what did I pierce with that needle and is that valve now fubar?
 

FrankieD61

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On my 76 Pontiac, one of the rear lines busted wide open and this caused the valve inside of the metal body to be forced to the rear port; stopping any fluid flow to the rear brakes. Most prop valves that GM used have a "reset" button that rarely works. This made the bleeding process pretty interesting. I even tried to do what you did and that didn't work. I finally had enough, got in the car and started SLAMMING on that pedal like it owed me money; I got it free. I was so relieved when I heard those rear brakes actuating again!

I THINK what you did was just move that valve to (hopefully) the center-most position; restoring fluid flow to the front and rears. Somebody who is actually a mechanic will chime in!
 

FrankieD61

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On my 76 Pontiac, one of the rear lines busted wide open and this caused the valve inside of the metal body to be forced to the rear port; stopping any fluid flow to the rear brakes. Most prop valves that GM used have a "reset" button that rarely works. This made the bleeding process pretty interesting. I even tried to do what you did and that didn't work. I finally had enough, got in the car and started SLAMMING on that pedal like it owed me money; I got it free. I was so relieved when I heard those rear brakes actuating again!

I THINK what you did was just move that valve to (hopefully) the center-most position; restoring fluid flow to the front and rears. Somebody who is actually a mechanic will chime in!

Forgot to mention that my dad blew the line... 20 some years before that. So the valve was quite happy where it was (in the "tripped" position).

Question for the mechanics on here ( I'm not a mechanic. I'm a hobbyist/enthusiast/just been doing this way too damn long type of person): what is the difference between a combination valve and prop valve? I'm no brake expert; I know what's safe and not safe. And basic working theory.
 

Schurkey

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Proportioning valve is in the rear hydraulic system ONLY. Has ZERO to do with the front brakes. Provides something of a limit to the hydraulic pressure delivered to the rear compared to the front, after a certain pressure is reached.

Combination valve is a metering valve (Front brake ONLY), safety switch, and proportioning valve (rear brake ONLY) in a single housing.

The safety switch moves out of the centered position when there's unequal brake pressure coming from the master cylinder, which trips the warning light on the dash. Most GM safety switches are self-centering. I've had to manually re-center two Ford safety switches, and it's a pain in the asp. I have never had a safety switch block fluid flow.

If you had problems getting fluid flow to the FRONT brakes, you almost certainly had a problem with the metering valve. They're supposed to "breathe" at zero-to-very-low pressure, seal at medium pressure, and open at higher pressure. I don't know how the front brakes could have been working based on your description of having no fluid flow when bleeding. Did the vehicle pull to one side, or were both front brakes getting no fluid?

Old diagram of a combination valve--this is probably an early-'70s design. (upper right side of page.)
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ericg75

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The truck hasn't run in over 20 years and I don't really remember how the brakes performed then. The lines were pretty well rusted through 10 years ago and I installed the new ones about a year ago.
 

Schurkey

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Make sure you have a high, firm pedal before driving it. As you drive, be sure to notice improper braking action--pulling to one side, low braking power, overly-sensitive brakes, etc.

You might want to verify brake temperature with an infra-red thermometer after a hard stop. Make sure both front brakes are about the same temp, and both rear brakes are about the same temp; and that they're all "hot".

Any wheel that significantly hotter or colder than the others needs attention.
 

ericg75

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Make sure you have a high, firm pedal before driving it. As you drive, be sure to notice improper braking action--pulling to one side, low braking power, overly-sensitive brakes, etc.

You might want to verify brake temperature with an infra-red thermometer after a hard stop. Make sure both front brakes are about the same temp, and both rear brakes are about the same temp; and that they're all "hot".

Any wheel that significantly hotter or colder than the others needs attention.
Thanks Schurkey. I've been looking for a reason to buy one of those IR thermometers. Between brakes and checking various header temps on the engine rebuild I think I now have a good excuse to pull the trigger.
 
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