Brake Problem
 

This is a 1973 C20 that I picked up recently . The front discs apply but the rear drums free wheel with the brake pedal hitting the floor . I replaced the master cylinder and bled the entire system but it is still the same . I have not removed the rear drums ( full floater/ tough ) because I am now thinking that it's the combination valve . How should I trouble shoot at this point ? how do you test a combination valve . Thanks .


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Re: Brake Problem
 

Are you sure its not the proportioning valve? Never heard of the combination valve. Not sure how to test it, maybe just take it off and clean her up. You said that you bled the brakes? Did fluid come out of the rear bleeder valves? If the pedal goes to the floor I would look at the master cylinder. Even if the proportioning valve was bad I don't think the pedal would go to the floor.

Re: Brake Problem
 

There seems to be some confusion on what the valve is called . I myself have used porportioning valve but my repair books and some other people call it a combination valve. I believe the combination term is the one used most . Anyways like I posted above the master cylinder was replaced with a new one and yes it bleeds lots of fluid at the rear drums when I crack the fittings . I think you are right that if the valve was bad the pedal would not go to the floor every time , so I/m back to square one . Thanks .


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Re: Brake Problem
 

How are you checking this? Do you have one side of the truck jacked up or are you taking the front brakes off? Because if you are takling them off then you will not build enough pressure to engauge the rear.

Re: Brake Problem
 

the truck is on jack stands with the wheels off and the brake system is fully assembled . With someone pumping the brakes the front calipers lock up the front rotors . The rear drums continue to free wheel with no resistance and the brake pedal will not build up and stays weak , no matter how much you pump it . Again the new master cylinder reacts same has the original one and fluid bleeds freely from all four corners . ??:confused: ??. Thanks


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Re: Brake Problem
 

i have seen a couple of people on here that ended up replacing the new master cylinder with another one,,dont know if thats your problem or not,but it has happened..:)