72 proportioning valve - The 1947 - Present Chevrolet & GMC Truck Message Board Network

hardhead · 09-23-2012, 09:21 AM

I know this should go under the brake forum but I'm not getting any response over there. I have a proportioning valve from a 72 and it doesn't have a button to re-center the shuttle valve in it, it just has a plug in the end. Anyone had a problem with one of these? how do you re-center it?

sjarrett71 · 09-23-2012, 09:57 AM

Remove the plug. The button should be under it. To recenter mine, I just stomped on the brake peddle a few times until it "popped" back into place.

geezer#99 · 09-23-2012, 10:07 AM

I found this. Did you do a search?
http://67-72chevytrucks.com/vboard/s...d.php?t=336053

hardhead · 09-24-2012, 09:34 AM

Tried all that and installed another valve. I think the new MC is bad on the front side. Going to replace it today and see if it fixes the problem.

**lolife99** · 09-24-2012, 09:43 AM

Do you have any pics of your M/C set up?

hardhead · 09-24-2012, 12:50 PM

Don't have a pic, I'm at work right now.

davepl · 09-24-2012, 01:11 PM

Hmm... I have the same valve, though brand new, since I'm converting over to '72 discs. I was not aware there was any magic in terms of setting these up, I thought you just plumbed them and bled them.

I'm guessing there is a shuttle valve that can stuck to one end and cause problems, but beyond that I have no idea.

Would someone be so kind as to tell me what it is that I don't know? I'll refer to the shop manual when the time comes, but helps if I know in a general sense what's going on.

Sort of like finding out one day that there are muffler springs and you had no idea!

hardhead · 09-25-2012, 12:10 PM

The proportioning valve has a centering valve I believe it is called. This valve is in case you severe a brake line or blow out something the pressure from the good side of the brake system will force it in the opposite direction to cover the port on the low pressure, or blown out side so you still have some brakes. I have never had this issue while bleeding a brake system before but until the system is bled and the pressure is equal on each side there is the possibility of this happening. There is a tool designed to go in place of the brake switch while bleeding to prevent this from happening, I found a picture of one on the net. I seem to have it fixed, not sure what it was changed the MC and cleaned the prop valve.

09-23-2012, 09:21 AM	#1
hardhead Registered User Join Date: Nov 2010 Location: saraland al Posts: 102	72 proportioning valve I know this should go under the brake forum but I'm not getting any response over there. I have a proportioning valve from a 72 and it doesn't have a button to re-center the shuttle valve in it, it just has a plug in the end. Anyone had a problem with one of these? how do you re-center it?

09-23-2012, 09:57 AM	#2
sjarrett71 Registered User Join Date: Dec 2006 Location: Rolla, MO Posts: 2,463	Re: 72 proportioning valve Remove the plug. The button should be under it. To recenter mine, I just stomped on the brake peddle a few times until it "popped" back into place. __________________ -Scott-

09-23-2012, 10:07 AM	#3
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,546	Re: 72 proportioning valve I found this. Did you do a search? http://67-72chevytrucks.com/vboard/s...d.php?t=336053

09-24-2012, 09:34 AM	#4
hardhead Registered User Join Date: Nov 2010 Location: saraland al Posts: 102	Re: 72 proportioning valve Tried all that and installed another valve. I think the new MC is bad on the front side. Going to replace it today and see if it fixes the problem.

09-24-2012, 09:43 AM	#5
lolife99 67-72 parts collector,… Join Date: Nov 2002 Location: Mid-MO Posts: 22,692	Re: 72 proportioning valve Do you have any pics of your M/C set up? __________________ Keith Convert to disc brakes. http://67-72chevytrucks.com/vboard/s...d.php?t=444823

09-24-2012, 12:50 PM	#6
hardhead Registered User Join Date: Nov 2010 Location: saraland al Posts: 102	Re: 72 proportioning valve Don't have a pic, I'm at work right now.

09-24-2012, 01:11 PM	#7
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: 72 proportioning valve Hmm... I have the same valve, though brand new, since I'm converting over to '72 discs. I was not aware there was any magic in terms of setting these up, I thought you just plumbed them and bled them. I'm guessing there is a shuttle valve that can stuck to one end and cause problems, but beyond that I have no idea. Would someone be so kind as to tell me what it is that I don't know? I'll refer to the shop manual when the time comes, but helps if I know in a general sense what's going on. Sort of like finding out one day that there are muffler springs and you had no idea! __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

09-25-2012, 12:10 PM	#8
hardhead Registered User Join Date: Nov 2010 Location: saraland al Posts: 102	Re: 72 proportioning valve The proportioning valve has a centering valve I believe it is called. This valve is in case you severe a brake line or blow out something the pressure from the good side of the brake system will force it in the opposite direction to cover the port on the low pressure, or blown out side so you still have some brakes. I have never had this issue while bleeding a brake system before but until the system is bled and the pressure is equal on each side there is the possibility of this happening. There is a tool designed to go in place of the brake switch while bleeding to prevent this from happening, I found a picture of one on the net. I seem to have it fixed, not sure what it was changed the MC and cleaned the prop valve.



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