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Discussion Starter #1
Back story:

2004 Ram 1500 short bed, standard cab. 2wd, manual transmission.
Truck only has 40k miles, and EVERYTHING except the tires and filters is original.

I go to pick up a load of mulch last weekend, and the brake pedal basically is so spongy that I can bottom the pedal out without locking up the brakes at any speed.

My first thought that was the fluid was so old, some moisture must have created a huge bubble in the system. I completely flushed/bled the system yesterday, zero change in performance.

If I stand on the pedal after all the vacuum in the booster is "used up" (after shutting off the engine), there is maybe some slight "bleed off" (perceived, at least... it may just be in my mind), but I can't get it to bottom out.

No leaks anywhere, cant find any bulges in the hoses, either. Could the MC be bypassing? Any procedure for diagnosing the MC and/or RWAL valve?

Some answers before the questions are asked: Pads and rotors are in good shape. Pedal mechanical linkage is good. No strange noises other than booster "whooshing" which is normal.
 

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You can rule out the MC if you make sure that the air is all bled out. Then cap the outputs. If the pedal is now rock hard then it isn't the MC.
 

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Discussion Starter #4
At that point, I'd probably just replace the MC.... As the cost of the new one is low compared to the effort required to re-bleed the whole system....

Any idea on diagnosing the system without taking it apart?
 

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Discussion Starter #5
Hmm, just found this response (on a cummins forum) regarding checking the RWAL valve to see if it's bypassing into the dump reservoir:

"If you are suspecting a failed dump cartridge valve you can test the LPA (low pressure accumulator) for fluid. They put these little rubber boots over the inspection ports on the end of the valve. What you do is take a Q-tip and put it in the little inspection port and go hit the brakes. If it pushes the Q-tip out then you have a Dump Leak. You can replace the valve with a remaned one or you can take a huge magnet and put it over the dump cartridge and try to bleed some fluid out of around it. The magnet will hold it open and you can work fluid/contaminants in and out of the valve much easier than conventional brake stomp activation then bleed. I work for the company that makes those little guys so if you have any more questions, ask away. "

I found that little inspection port and was wondering how I could use it to my advantage....
 

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Also check the rubber lines to the caliper to see if they expander under braking conditions, someone presses pedal and someone else looks at line.
 

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Discussion Starter #7
Yep, I was on my back under the truck while my wife stood on (and off) the pedal... couldn't detect any bulges/balloons/elengation.
 
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