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Tractor Talk Discussion Forum

1979 GMC C7000 - replacing master brake cylinder

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andy r

06-12-2018 22:12:26




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Truck is a 1979 GMC C7000 with a single axle on back with duals. Engine is a 366. Truck has a booster behind the drivers side front wheel. Truck hauls short of 400 bushel. I think my master brake cylinder is leaking some. Looks as if the firewall is wet inside and out and the master brake cylinder does loose oil over time. Searching I find a part number 10-57577 with a 1 3/4 inch bore and a 10-57579 with a 1 1/2 inch bore. Anyone know if they are basically the same. Could I use a 1 3/4 inch bore even if the truck calls for 1 1/2 inch bore. Takes two pumps of the brake pedal to get good brakes. Maybe the 1 3/4 inch bore would help to fix that problem. Thanks.

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Joe(TX)

06-14-2018 20:33:03




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
The size of the bore has nothing to do with your problem. increasing the bore size will increase the required pedal pressure.
Your problem is most likely due to air in the system or a fluid leak. Use the same size as you have.



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riveroadrat

06-13-2018 21:17:20




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
A new master cylinder doesnt cost that much. I have a new one but it has a little rust inside the reservoir from sitting. It would probably need to be taken apart and de-rusted with evaporust. Your master should have one line going to the hydrovac. You can check to see if the hydrovac is leaking by removing the pipe plug on the backside of the diaphragm housing and stick a long piece of elecrical wire in that plug hole to see if it has brake fluid in it. Use the wire like a dipstick. We had quite a few of those trucks and never had to replace any wheel cylinders on any of them.. Brake fluid likes to soak up moisture so its a good idea to change the fluid every once in a while. The hydrovac works just like a power brake booster on a car, it uses engine vacuum. I know that little emergency brake dont do much even on an empty truck. It sure beats no hope at all.

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hd6gtom

06-13-2018 17:23:01




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
Andy I owned several of those trucks. If you have to pump the brakes then you need to adjust them. If they get too worn before you adjust them, they will not stop no matter how much you pump them. Please stay with the same master cylinder. New ones at NAPA are not that much.



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used red mn

06-13-2018 10:38:27




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
Their is a possibility the larger bore master cylinder is for a truck with tandem rear axles. I would recommend staying with stock dimensions as well. Pull it out tear it down to rebuild then you know what size you have. The previous post Iím attaching has a lot of info in it on causes of needing to pump the brakes or low pedal. Hope you find the world of happiness that has bleeders that arenít rusted tight. Good luck!

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used red mn

06-13-2018 10:40:07




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to used red mn, 06-13-2018 10:38:27  
Didnít even realize it was a reply to your earlier post. Lol!



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David G

06-13-2018 09:10:08




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
Those master cylinders can be rebuilt fairly easy IF the bore is not pitted too badly.



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INCase

06-13-2018 07:00:55




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
As others have noted. I would not change the brake system. Replace with what is on it now presuming that is what is came with and designed for from the factory.

does it have hydro boost (gets hydraulic pressure from PS pump to boost) or Electric boost?

lots of things to check and make sure are working correctly on these medium/heavy trucks. its not air brakes so if you loose the system you'll have no brakes.

i imagine its like alot of old grain trucks and has next to no miles on it. Age becomes your enemy after a while regardless of miles. gaskets/bushings and seals can deteriorate and go bad.

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David G

06-13-2018 06:23:05




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
Mark B brought up a good point, with larger diameter you will apply the brakes sooner, but it will take much more pedal pressure to stop.

1 1/2 inch = 1.76 inch squared
1 3/4 inch = 2.40 inch squared

so, required pedal effort will be 1.4 times higher



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Bob

06-13-2018 04:15:49




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
"Takes two pumps of the brake pedal to get good brakes."

You will be thoroughly bleeding the brakes after replacing the master cylinder, then "set up the brakes", and I think that problem will go away. Don't forget there's a bleeder on the hydrovac.

There's different size mater cylinders because the brake slave cylinders at the wheels vary in size, as well, due to axle weight rating variations, or maybe who made the particular axle/wheel/brake setup used on your specific truck.

Also, it wouldn't be uncommon for the hydrovac to be leaking brake fluid, either, into the engine's air intake. Take the vacuum hose off of the hydrovac and note if there's any wetness/brake fluid inside of it.

Also, it's a good time to check/clean/replace the hydrovac exhaust filter, likely cleverly hidded up behind the driver's step, or possibly behind the seat.

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MarkB_MI

06-13-2018 03:04:25




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
I wouldn't recommend it. Yes, it will deliver 36 percent more volume per pump. But it will also deliver 27 percent LESS pressure to the brakes for the same pedal effort. Not what you want when you're hauling 400 bushels of grain.

I'll bet your pumping issue goes away after you replace the master cylinder. If it doesn't, check the brake shoes; they may have run out of adjustment.



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CCer

06-13-2018 02:46:56




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to andy r, 06-12-2018 22:12:26  
Years ago, the parts counter had a book called a buyer's guide. It listed all the specifications/dimensions of each part number. I used that book to compare parts, as you would like to do. Good luck!



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504

06-13-2018 21:45:37




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 Re: 1979 GMC C7000 - replacing master brake cylinder in reply to CCer, 06-13-2018 02:46:56  
You do not want the extra work but pull the drums and check all four wheels to make sure the wheel cylinders are dry and make sure they are not stuck. After inspection you will know whether it is safe to drive after you replace the master cyl. The pedal pump is caused by brakes that need adjustment.



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