How do you tell if a Brake Combination Valve is Defective? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72 GMC K1500 · 01-18-2006, 01:49 AM

Can anyone tell me how you can diagnose a combination valve? I have replaced almost ALL of the front and rear brake components on my truck but the brakes are still terrible! I get very little nose dive on the truck when the brakes are applied with a lot of pressure and my only thought was that the metering valve component of the valve may be defective. Any thoughts?

71/454 · 01-18-2006, 02:19 AM

Just my 2 cents but, I had an 81 do this same thing, you practically had to stand on the pedal to make it work. I went thru and changed a bunch of stuff and ended up living with it. I sold the truck to a friend who drove it into the ground and never cared to fix anything. I ended up buying it back a year or two later and as soon as I drove it I knew it needed a booster. I guess it was just slowly on it's way out when I had it and he could have cared less. When I was trying to diagnose it, it still held vacum if u pulled off the hose and the pedal sunk when u started it. I guess I'm trying to say maybe it's the booster, you never can tell seems like!

avejoe · 01-18-2006, 02:25 AM

Been there, done that.

Replace your booster with a dual diaphram booster. It'll put you through the windshield.

72 GMC K1500 · 01-18-2006, 02:28 AM

Thanks. I have not yet replaced the booster. Like you mention, mine does hold vacuum and I stopped looking there. Do you know how to test a booster other than checking for leaks at the hose and fitting?

71/454 · 01-18-2006, 02:46 AM

Like i said it didn't work for me but u might as well try it. Sit in the truck with the engine off apply the brake fairly firm and then start the truck, if it is working properly the pedal should drop when the engine starts. I'm not sure this is an official troubleshooting method, I learned it as one of those "grandfather" teachings in a shop I used to work at.

72 longhorn BB · 01-18-2006, 08:13 AM

I just googled your symptom, and found this link:

http://autorepair.about.com/cs/troub...g/a/bl302a.htm

hope this helps

72 GMC K1500 · 01-18-2006, 10:26 AM

Will the rest of my brake components work with a dual-diaphram booster? Or will I have to replace (again) the Master Cylinder? Is the dual-diaphram booster a 3/4 ton unit, or for a later year truck? The reason I am asking is when I go to NAPA I just want to ask for the correct part. Thanks.

avejoe · 01-18-2006, 11:31 AM

You will need to make sure the MC is designed to work with the new booster. The push rods can be of different length.

01-18-2006, 01:49 AM	#1
72 GMC K1500 Registered User Join Date: Mar 2005 Location: Castle Rock, CO Posts: 24	How do you tell if a Brake Combination Valve is Defective? Can anyone tell me how you can diagnose a combination valve? I have replaced almost ALL of the front and rear brake components on my truck but the brakes are still terrible! I get very little nose dive on the truck when the brakes are applied with a lot of pressure and my only thought was that the metering valve component of the valve may be defective. Any thoughts? __________________ 1972 GMC K1500, sbc 350, turbo 350 Castle Rock, CO Last edited by 72 GMC K1500; 01-18-2006 at 01:50 AM.

01-18-2006, 02:19 AM	#2
71/454 Turbo Guy Join Date: Jan 2005 Location: Calgary Alberta Posts: 353	Re: How do you tell if a Brake Combination Valve is Defective? Just my 2 cents but, I had an 81 do this same thing, you practically had to stand on the pedal to make it work. I went thru and changed a bunch of stuff and ended up living with it. I sold the truck to a friend who drove it into the ground and never cared to fix anything. I ended up buying it back a year or two later and as soon as I drove it I knew it needed a booster. I guess it was just slowly on it's way out when I had it and he could have cared less. When I was trying to diagnose it, it still held vacum if u pulled off the hose and the pedal sunk when u started it. I guess I'm trying to say maybe it's the booster, you never can tell seems like!

01-18-2006, 02:25 AM	#3
avejoe Custer had a plan too. Join Date: Dec 2003 Location: No. California Posts: 344	Re: How do you tell if a Brake Combination Valve is Defective? Been there, done that. Replace your booster with a dual diaphram booster. It'll put you through the windshield. __________________ 72 K15 GMC (aka Big White) Performance Rockers, Cab Corners, and Floors Fossil Fuel Glutton Neighborhood Nuisance 98 BMW 540i for Speed (Sold ) 05 Subaru Outback for mileage

01-18-2006, 02:28 AM	#4
72 GMC K1500 Registered User Join Date: Mar 2005 Location: Castle Rock, CO Posts: 24	Re: How do you tell if a Brake Combination Valve is Defective? Thanks. I have not yet replaced the booster. Like you mention, mine does hold vacuum and I stopped looking there. Do you know how to test a booster other than checking for leaks at the hose and fitting? __________________ 1972 GMC K1500, sbc 350, turbo 350 Castle Rock, CO

01-18-2006, 02:46 AM	#5
71/454 Turbo Guy Join Date: Jan 2005 Location: Calgary Alberta Posts: 353	Re: How do you tell if a Brake Combination Valve is Defective? Like i said it didn't work for me but u might as well try it. Sit in the truck with the engine off apply the brake fairly firm and then start the truck, if it is working properly the pedal should drop when the engine starts. I'm not sure this is an official troubleshooting method, I learned it as one of those "grandfather" teachings in a shop I used to work at.

01-18-2006, 08:13 AM	#6
72 longhorn BB Registered User Join Date: Jan 2000 Location: Michigan Posts: 312	Re: How do you tell if a Brake Combination Valve is Defective? I just googled your symptom, and found this link: http://autorepair.about.com/cs/troub...g/a/bl302a.htm hope this helps __________________ Chuck 1972 C-20 Longhorn 402 BB .030" over (409?), curved HEI, RV cam, quadrajet, dual 3" exhaust, TH 400 ported w/shift kit 1999 K1500 Suburban (old style) 5.7L Vortec 4L60E "No matter where you go, ........ there you are"

01-18-2006, 10:26 AM	#7
72 GMC K1500 Registered User Join Date: Mar 2005 Location: Castle Rock, CO Posts: 24	Re: How do you tell if a Brake Combination Valve is Defective? Will the rest of my brake components work with a dual-diaphram booster? Or will I have to replace (again) the Master Cylinder? Is the dual-diaphram booster a 3/4 ton unit, or for a later year truck? The reason I am asking is when I go to NAPA I just want to ask for the correct part. Thanks. __________________ 1972 GMC K1500, sbc 350, turbo 350 Castle Rock, CO

01-18-2006, 11:31 AM	#8
avejoe Custer had a plan too. Join Date: Dec 2003 Location: No. California Posts: 344	Re: How do you tell if a Brake Combination Valve is Defective? You will need to make sure the MC is designed to work with the new booster. The push rods can be of different length. __________________ 72 K15 GMC (aka Big White) Performance Rockers, Cab Corners, and Floors Fossil Fuel Glutton Neighborhood Nuisance 98 BMW 540i for Speed (Sold ) 05 Subaru Outback for mileage



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