Proportioning valve could be acting up, either that or there's some air in the lines someplace. The brake light coming on is from the shuttle cock in the proportioning valve sliding from side to side because of the pressure differential ex. If you blew a brake line or air in the line. Did you replace the hardware? Weak springs can mess with the timing of the brakes, rear is supposed to apply slightly before the front to reduce the possibility of the rear end trying to pass the front during hard braking.


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How far does your pedal move? Might even be an air locked or faulty master cylinder


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it goes to the floor on start up then when im driving its half way down but when the light comes on i cant push the pedal down

Vacuum problem... Our trucks have a vacuum switch that lights the brake light up when we lose vacuum. My pumps are weak and my light will come on/stay on and I have less than desirable braking assist at times.


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Sounds like the booster is leaking causing the hard pedal, soft/mushy pedal is prob the dump valve/ or air/ or a bad master or a combo of those. On my truck the dump valve went bad, the soft pedal to the floor caused the booster to go bad as well from over-extending the travel of the brake linkage. I fixed (read removed) the dump vale but still get a bit of a vac leak/hard pedal if I sit at a stop light for too long.

where should i start checking for a vacuum leak?

Check the pump itself first, unhook it from the booster and put a gauge on it... Or see if you have any vacuum at all. But it seems you have vacuum on the initial start up, so I'd check when the problem is present


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ok thanks for all the advice im going to do the booster, proportioning valves and bleed the shit out of them and new rears.

I had a bolt break between the master Cylinder and the Booster a while back and it made for a really stiff pedal cuz the booster was trying to push the master cylinder right off... However the brakes still worked if you pushed hard enough so its probably not the same situation here.

Also, tower_ofpower knows what he's talking about and like he said, Gas apparently doesn't quite know, at least as far as the M70 goes. The rear bearings sit in diff fluid for a reason, and bad/low oil like you mentioned in the past may very well have caused them to go bad, in which case replacing them is probably a good idea, and they're not very expensive and now would be the time. Once you replace them, its also a good reason to put some good oil in there... Ive gone with RP diff fluid before in a Jeep and although I can't say ive seen a major improvement, I can definitely say I haven't seen a problem with it since. The high pressure comment about the oil is completely true and one reason RP is just as good a choice on a diesel as it would be on a porche, because we push a lot more pressure than anybody else, and keeping those teeth from contact is just as important as keeping the oil in your motor... Also, the agreement from 93firstgennewbie right after the royal purple comment can be discredited as well because as BC847 mentioned, all Dana 70s are full float, which you can see just from looking at the hub from the outside. I may be new to Cummins and Dodge, but being a Jeep guy for forever, I know my Dana/Spicer, and I definitely see who else is with me on that one... Ive built axles from a housing and some DOM tubing, to a complete crawler axle, and It definitely sends off red flags to me when I hear something that doesn't add up (like packing D70 wheel bearings), and I see tower_ofpower and BC847 know what I mean. They're also right about the tablespoons of fluid leaking out and you may want to check the level when you get everything done but you shouldn't lose much.

Anyway, like previously mentioned, try to keep from cutting people down unless you really know your stuff, cuz once you bring out that crap, somebody's gonna prove to you that you don't. and if you're gonna find the experienced guys that can, its on this forum. I know that because ive asked questions on here and had an overwhelming amount of knowledge flow out of some of these guys.