Posted by JD Seller on November 30, 2015 at 23:01:05 from (208.126.198.123):
In Reply to: Help with semi brakes. posted by notjustair on November 30, 2015 at 22:40:45:
I know this will sound like a dumb question but have you adjusted the brake slack adjusters???? It sounds like the brakes are just way out of adjustment.
If you have adjusted the brakes then there is one other explanation. The brake valves on the rear axles on a semi have a special feature built into them. What it does is makes it to where you can't apply air pressure/force and the parking brake spring pressure/force both to the brakes at the same time. So IF you have the parking brakes set, which actually dumps the air that releases the spring to apply the brakes. If you push on the brake pedal which applies the service brakes this valve releases the parking brakes and applies air to the service brake diaphragm. So if your hearing the brakes work while the parking brake is applied the valve is not working correctly. This valve is usually mounted right around the middle of the front drive axle. Then the front and rear brakes are teed together. With parking brakes on one section and service brakes on the other.
This function will do funny things when it quits working correctly. Like releasing the parking brakes but not applying the service brakes. That is what it sounds like is going on.
To check it take an air line off the service brake side of one of the rear brake chambers and see if you have air flow with the parking brakes set with the service brake/foot brake pushed. If you do not then the valve is not working.
Email me if you want to talk about it in person. I don't know if I explained it to where you can understand what is going on. So shot me an email if you have any questions.
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