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Bleeding brakes on my GMC


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I don't mean bleeding as in *&&^%$@!? brakes but actually meaning to get the air out of them.


Ok here it is the situation, manifold done! UJ done! Brush Guard bolted back on! Lets go for a road run I say to Jess (Jack Junior) fire the beast up, pull out of the drive......can't stop, no brakes :evil: :shock:


So what has happened there?


I have replaced the brake switch as the last one was worn out and when I undone the switch a small amount of brake fluid ran out. Is this the problem?


Is this the sump plug to the Master Cylinder and the little bit of oil that did run out was the last drops left?


The breaks up to this point where fine and well balanced.


Can someone please HELP.





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Jack.... refill the master cylinder again. put a few drops of engine oil in the vacuum chamber of the hydrovac.

Then start the engine.... you will see that the majority of air will come back to the resorvoir of the mastercyl.

then bleed them the old fashion way.... pressing the pedal few times. hold it under pressure when there is resistance and a helping hand bleeds the brakes one by one starting with the longest line from the mastercyl.

keep the resorvoir full at all times.


The thing that happened to your truck is that a small amount of air came in the line to the hydrovac cylinder. this causes the brakes not to work.

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Hi Jeroen.


Thanks for that mate, but where is the bleed screw on the Hydrovac as Joel was saying that you have to bleed this first?

there are 3 types of hydrovac used on the GMC and even an booster on the early ones.

Not all of the vacs have a bleedplug... Because of the high pressure that the hydrovac is creating on the brakesystem, you only have to wait a few minutes and then bleed the wheelcyl.





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Thanks guys.


I filled up the MC and watched the air bubble out. I then went to the back axle and bled the far right and had a hard peddel right away. I have road tested it over the last 2 nights up and down hill, high and low speed and also under hard pressure when a car decided to try and stuff itself into the front of the truck :shock: breaking so far hasn't shown any side effects.....


.........is this to good to be true?

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yeah I think you are right gents, put it this way, the road run that we went on during the weekend involved climbing up one of the steepest hills in the Purbecks, it pulled up there a treat and we went down the other side on the brakes and with the truck full of Germans........no problems. I did try and tell them that I was having problems with the brakes but I think they thought that I was joking :shock: :shock: :wink:

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