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BucketTrucker688

Relay Valve? Not able to feather into brakes??

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Would a relay Valve be the culprit for having nothing happen for the top 1/4 of my brake pedal? I have read a sticky relay Valve will show the symptom of not being able to feather into the brakes but I have 3 relay valves and don't know how to diagnose which it may be. The truck is a 91 rd688s Tri axle dump. I was going to change the foot Valve until I read it may be a relay Valve which looks easier to change than a foot Valve. This makes for a white knuckle ride in the rain when all the brakes want to do is lock up on you and you slide to a stop every traffic light.

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Here is a link to a brake handbook.

http://www.suspensionspecialists.com/techinfo/Bendix_Brake_Handbook_2009.pdf

A relay may be bad or some crud is in the system so it is causing issues.  First thing I would check is if the roller on the back side of the brake pedal is moving smoothly.  Is the plunger that the roller contacts in the brake pedal base plate able to move smoothly in its bore.  If there is an issue with either, clean them up and see how the brakes apply.

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Did this just suddenly happen?... are all axles locking up at the same time or is it just one?... have you checked the brake adjustment and watched the S-cam action as someone operates the brake...

go with what jhancock advises, in your work the driver brings sand and gravel into the cab on his boots and if the rubber boot is worn/split it gets into the top of the foot valve... remove the plunger and roller, clean, inspect and lube and also check the pins for binding.

if you have a brake application valve on the dash watch it as you are braking, does it go from zero to 20+ in a jump?, you should be able to feather it to give 5psi then 10psi...

you probably have a relay valve for each axle, so if a relay is bad it will only cause issues on that one axle.

BC Mack

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