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w896andy

GMC 353 Brakes lock on, some advice please !

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I've left my GMC CCKW 353 standing over the winter and now find the brakes lock on with the engine running, but release when its not. The brakes have a nice amount of travel without the engine running, start the engine and after the first brake application the brakes lock on, the pedal goes very hard with no movement. They stay on and only releases after I stop the engine when the vacuum dies down and the pedal then has travel again. I suspect the Hydravac system but can't work out what the likely cause is. Can the relay valve stick some how on the end of the hrydravac ? If so how do I test it or where do I get a replacement ?. I've checked the master cylinder, fluid level and pedal adjustments and the master cylinder seems to release ok. I've attached a picture of the unit as I belive there are different ypes.Any help gratefully received, thanks.

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Hi, I have never had this problem with my GMC, but I think there has been past conversations about this issue. From what I can remember you need to make sure there is enough free play on the adjusting rod to the master cylinder, if not the piston in it can cover the fluid return hole back to the reservoir in the master cylinder. This then applies pressure in the hydrovac and when engine started it thinks you are pressing the pedal and so as vacuum rises jams brakes on.

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Also check your brake hoses. If the bore of the pipes is collapsing it will give you all sorts of odd symptoms. They may not even show any signs of deteriorating on the outside.

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Thanks both. It was the Hydravac itself that was faulty. I took the unit off and took the regulator valve off the back, the insides were full of grease and this appeared as well as other things to have blocked up the air bleed in the valve casing.  The vehicle was a french rebuild, stored for many years until I bought it. After I did some miles the brakes got worse so it looks like the grease inside gummed it up and kept the brakes on as soon as there was vacuum pressure. I took the safest route and got a newly rebuilt unit and replaced it rather than risk just cleaning out the unit.

 

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I have the same problem, but only the front drivers side wheel locks up, If i loosen the bleed nipple it frees up and is ok for a while.

 

I'm suspecting the brake hose, but thismake sure there is enough free play on the adjusting rod to the master cylinder, if not the piston in it can cover the fluid return hole back to the reservoir in the master cylinder. This then applies pressure in the hydrovac and when engine started it thinks you are pressing the pedal and so as vacuum rises jams brakes on" may be an issue too.

 

 

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