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1977 GMC 6500 Brake questions

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Cariboo4x4
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PostPosted: Wed Jun 17, 2020 7:12 am    Post subject: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

Hi All

I am trying to figure out why the vacuum brake booster no longer works on this 1977 GMC 6500. Ive followed the vacuum line to the vacuum booster under the cab to this valve as I call it under the slave cylinder. The slave cylinder is bolted to 2 vacuum pots as I call them. I have yet to crack any of the rusted lines. We have bled the master cylinder and slave cylinder yet the vacuum assist function appears to be not working. I have taken lots of photos. Ive attached a few.
 
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Cariboo4x4
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PostPosted: Wed Jun 17, 2020 7:15 am    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote


 
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Cariboo4x4
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PostPosted: Wed Jun 17, 2020 7:19 am    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

I normally use Photobucket to load my photos or modify their size. Ill try to post photos over the next few days. Thank you for any input you can assist with my vacuum question. The vacuum booster set up under trunk I dont know what the exact name its called to order it if needed or if the valve that the vacuum line attaches to needs replacing or rebuilding only. Thank you in advance to any suggestions.
 
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Nordic
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PostPosted: Sat Jun 20, 2020 5:57 pm    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

I guess your pictures didn't upload but if you have vacuum coming to the booster and it's still not working the diaphragm in the booster is likely ruptured. I had to change one out for that reason but not on my 6500.
 
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Cariboo4x4
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PostPosted: Fri Jun 26, 2020 5:16 am    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote


Thank you. I just havent had time to load photos. Maybe later today.
 
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Swede in IOWA
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PostPosted: Wed Jul 15, 2020 2:04 pm    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

been trying to reply for several days. Those GM trucks have a history of the vac hose from the engine collapsing. Swede
 
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Cariboo4x4
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PostPosted: Thu Aug 06, 2020 2:39 pm    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote


Thank you! A late thank you yet Ill observe the vacuum hose when its running. Other projects too priority yet Ill get to this truck soon.
 
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PostPosted: Thu Aug 06, 2020 3:15 pm    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

Figured out how to load photos in a smaller size. One photo is of the double vacuum boosters. The other photo shows the slave cylinder attached to rear vacuum booster. I was wondering if the valve below slave cylinder is a valve that can fail. Im hesitant to start taking removing lines from valve or the square plug from booster. We oxy\acet for hear and propane. If there is vacuum coming to this vacuum booster set-up where does a person start and end to check if the boosters are working?







 
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Cariboo4x4
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PostPosted: Thu Aug 06, 2020 3:17 pm    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

Heres the valve. Ignore the receipt

 
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Cariboo4x4
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PostPosted: Thu Aug 06, 2020 3:33 pm    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

What is this vacuum booster set-up called?
 
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Swede in IOWA
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PostPosted: Sat Aug 08, 2020 10:12 am    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

Those hoses collapse on the inside cannot be seen on the out side , just have to pull it off and check for vacuum. Swede
 
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Jim in Sask
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PostPosted: Fri Aug 14, 2020 7:22 pm    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

Local mechanic that just overhauled the brake booster on my '83 Chev tandem told me when you take the booster off and shake it you can hear the brake fluid sloshing around inside if the diaphragm is shot. Said mine was about half full of oil. Rebuilt single boosters are generally available but mine has a double-chamber that had to be sent to Winnipeg to be rebuilt. They're less common than the single.
 
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PostPosted: Wed Aug 19, 2020 4:34 am    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

Jim in Sask wrote:
(quoted from post at 20:22:54 08/14/20) Local mechanic that just overhauled the brake booster on my '83 Chev tandem told me when you take the booster off and shake it you can hear the brake fluid sloshing around inside if the diaphragm is shot. Said mine was about half full of oil. Rebuilt single boosters are generally available but mine has a double-chamber that had to be sent to Winnipeg to be rebuilt. They're less common than the single.


Thanks Guys. All advice greatly appreciated!
 
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PostPosted: Fri Nov 13, 2020 11:48 pm    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

Well finally put truck in shop. I was wondering with truck running the vacuum gauge thats mounted in dash reads 15 psi. Vacuum drops to zero when I hit brake pedal. When foot comes off the brake pedal the vacuum comes back up to 15 psi. My thinking is that vacuum should stay at 15 psi regardless if your pressing brake pedal. Am I right?

We took pipe plug out of front of most forward brake booster. Buddy put his thumb over hole as I started truck and he could feel lots of vacuum. Again if I pressed brake pedal, buddy said he would not feel any vaccum. The forward section of both boosters where the vacuum lines attach should be sealed units and vacuum should constantly stay close to 15 psi even when pedal is pressed. Am I right?

Also I feel pedal drop slightly as I start engine. However as engibe runs and I push pedal a few times it feels hard, unlije the first time when I started truck with brake pedal slightly pressed.

I found out the truck uses the Cardone 51-8009 brake booster set up.
 
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Nordic
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PostPosted: Sat Nov 14, 2020 10:48 am    Post subject: Re: 1977 GMC 6500 Brake questions Reply to specific post Reply with quote

The pedal dropping slightly on start-up is normal. The system is definitely loosing the vacuum when the pedal is applied due to a leak somewhere. The quest is - at which point. Any and every point from supply to application is "suspect". Trouble shooting is sometimes very tedious... there is a rat hiding somewhere. Thanks for keeping us posted.
 
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