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4/29/11 6:53am - Original Message: 'proportioning valve question'
manchestershark Lifetime Member
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Equinunk, PA - USA

Vette(s):
1972 conv, 4-speed, 350, 200hp, numbers match, rally wheels, war bonnett yellow w/white top. good condition, nice driver.

Joined: 10/31/2007
Posts: 1901
 The rear feeder line has a what seems to be a union on the valve. I have been looking for a leak for awhile, and have replaced lines and still have a leak. I have attempted to tighten the so called union and only get flex in bracket even when I hold the valve. Do I have to replace the whole valve?, or is the union replaceable?  The catalogs do not show much, and only have the valve with union attached for sale, $110.00. I am going to check a local speed shop and see if they can locate one.
          ALSO, does anyone have a useable strut rod for the rear of a '75 they want to sell? I do not need bushings as I have a new set of poly ones ready to install. The car has two different size rods and have over size bushings in one rod. I want to put the same size on with the bushings I have. THANKS Much.
 
        B.T.W.  the valve is on the brake system.  Left front frame rail. NOT to insult anyones intellegence.   Thanks
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4/29/11 10:31am - Reply: 'proportioning valve question'
MikeMc71
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Tucson, AZ - USA

Vette(s):
1970 White L-46 Air Roadster, 1971 White LT1 Coupe, 1971 Red Auto Air Coupe, 383 stroker, 430 HP, 2002 Millennium Yellow Auto Coupe

Joined: 10/7/2007
Posts: 341
I have never seen a union style fitting on this valve (its actually just a switch, not really a proportionong valve, but everyone calls it that). If they have one for sale that way, I suppose that is what your car needs. If you are just looking for the union itself, I would check with a hydraulic fittings supplier. All those fittings are just commmon high pressure fittings, including all the brake lines.

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4/29/11 12:16pm - Reply: 'proportioning valve question'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
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Which car is this on Mark? The later cars like mine have a weird set-up like that, but the early cars didn't.

 


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Joel Adams
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4/29/11 12:28pm - Reply: 'proportioning valve question'
manchestershark Lifetime Member
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Equinunk, PA - USA

Vette(s):
1972 conv, 4-speed, 350, 200hp, numbers match, rally wheels, war bonnett yellow w/white top. good condition, nice driver.

Joined: 10/31/2007
Posts: 1901
Confused  '74-up,  Your picture also includes this female "union" for rear feeder line for rear brakes. It is my belief that the leak is between the "union" and the block. THANKS

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4/29/11 12:33pm - Reply: 'proportioning valve question'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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I think you can unscrew that fitting, and replace the seal(o-ring, or maybe a crush washer). There is a HEAVY spring behind it, tho...if memory serves. Adams' Apple2011-04-29 10:34:17

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4/29/11 2:05pm - Reply: 'proportioning valve question'
manchestershark Lifetime Member
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Equinunk, PA - USA

Vette(s):
1972 conv, 4-speed, 350, 200hp, numbers match, rally wheels, war bonnett yellow w/white top. good condition, nice driver.

Joined: 10/31/2007
Posts: 1901
 I could not budge it ON the car. I will try again, and hold it a little more firm.  THANKS

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4/30/11 8:29am - Reply: 'proportioning valve question'
tinbender
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edgwood, MD - USA

Vette(s):
1975 convertible,with the hard top and power streering.it a 350 L48 with auto trans. the color is light blue.

Joined: 11/1/2007
Posts: 736
When mine was leaking I couldn't get the leak to stop ,it was easier just to change the whole valve.Haven't had a leak since.The only problem I had was finding the valve,it seemed at that time no one had one.Good Luck.
          Rick


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4/30/11 11:11am - Reply: 'proportioning valve question'
kstyer Lifetime Member
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CUYAHOGA FALLS, OH - USA

Vette(s):
1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!

Joined: 12/2/2003
Posts: 6410
Its really a pressure differential switch.  If the system leaks front or rear, the switch will respond to the pressure difference and turn on the brake warning light.
 
A proportioning valve allows equal pressure front and rear up to a point, then only allows a portion of the pressure, a percentage, to go to the rear brakes to keep them from overapplying.  This is only needed with disc/drum brakes.   If it did not lessen the pressure as the rear of the car lifts during braking the rear becomes lighter and requires a lesser percentage of pressure to the rear to keep them from locking up.    Often trucks use a ride height sensor so the higher it lifts the more rear pressure it restricts.  With a heavy load on the rear it requires more braking, but does not lift as much with the load, and therfore allows more rear braking.
 
Any car with 4 wheel disc brakes does not need this.  Notice the rear caliper pistions are smaller then the front caliper pistions.  This takes care of the front/rear balance and the valve is not needed.  But you still need to know if you have a pressure loss at one end of the car, so we have the pressure switch.


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4/30/11 11:56am - Reply: 'proportioning valve question'
rraider1
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Woodland, WA - USA

Vette(s):
1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

Joined: 10/14/2003
Posts: 1152
Just to add if their is a bad enough leak it will set and shut off the bad section to keep from loosing all breaks. once it sets the brake light will remain on untill the section is fixed and bleed the hopefully it will reset

Proportioning valves provide four functions:

1. Proportion correct pressure to the front and rear braking systems.

2. Provide 10psi residual pressure in the brake lines.

3. Is a metering valve to apply pressure to the rear brakes before the front brakes.

4. Has a brake warning light to detect a loss in pressure.

rraider12011-04-30 10:02:40

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Bob R
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4/30/11 11:41pm - Reply: 'proportioning valve question'
kstyer Lifetime Member
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CUYAHOGA FALLS, OH - USA

Vette(s):
1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!

Joined: 12/2/2003
Posts: 6410
Bob thanks for the good info.  But the tech instructor in me come out to rear my ugly head, and I must add more.
 
The valve with 4 functions is a combination valve.  It contains the proportioning valve, metering valve, residual pressure valve, and pressure differential switch.   Granted it is very commonly refered to as a proportioning valve, but that's only part of it.  The functions listed in Bobs post are:
 
1. Already discussed above.
2. Holds residual pressure for two purposes.  First, to prevent the caliper pistons from creeping back in away from the caliper when driving.  Not a factor in our C3s due to the springs located behind the pistons.  With out the springs or pressure, the extra movement can cause a low brake pedal at times after cornering or driving some time without stepping on the brakes.    Second, by maintaining some pressure it ensures the caliper and wheel cylinder seals stay in contact with the surfaces and don't allow fluid out, or air in.  Not all designs need this, such as o-ring seals.
3. With drum brakes the fluid pressure must overcome the return spring pressure for the brake shoes.  This can take up to 75 psi in some cases.  Without the metering valve the front brakes would apply first, which can create poor brake performance.  They must apply evenly, or apply the rear first to keep the vehicle stable.  In many cases, 4 wheel disc do not need this function.  Without spring tension to overcome, the brakes apply evenly.  There are a few 4 wheel disc designs that still need this valve.
4. Again, already discussed above.
 
The reset can be tricky.  Once the repairs are made, the pressure diff valve may lie to you and keep the warning light on.   The switch works with a spool that remains centered with even pressure on the switch spool.  If one side drops pressure the higher pressure on the other side moves the spool and triggers the light contact, which creates a ground and turns on the light.
Some vehicles have a mild spring on each side of the spool to recenter it when pressure is relieved.  Others rely on bleeding the system to recenter the light.  Once the system is properly bleeded, one side or the other (front/rear) needs bleeded to push the spool back.  Open the bleeder, pressure drops and the spool moves.  Have someone push the brake pedal, even if a pressure bleeder was used.  Have the key on so the brake light is on.   As you are bleeding, have you helper watch the light.  When it goes out, he/she stops depressing the pedal, and lets you know to close the bleeder.    This can be done at the master cylinder.   Once everything is tight again, push the pedal a few times to be sure the light stays out.
kstyer2011-04-30 21:54:26

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(some day, no strike that, October 2008 it turned red, still in progress!)

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5/1/11 12:08am - Reply: 'proportioning valve question'
rraider1
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Woodland, WA - USA

Vette(s):
1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

Joined: 10/14/2003
Posts: 1152
Thanks for adding more info I thought it would make for good discussion

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Bob R
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