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treadle valve mr688s


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I have a quick question. I am working on a 2003 mack MR688s and the brake pedal is rock hard, not spongy at all like normal air brakes.

I've tried to clean the valve, but it doesn't help at all.

Is this valve pretty simple to replace? I talked to a mechanic and he said that you should replace the bracket with the valve because the brackets tend to break.

What does the valve look like when removed?

Mack told me the part was like $60.00, but the mechanic said the valves normally are $200-$300.00 so I may have told him to get me the wrong part?

I told him I needed a treadle valve for the brakes.

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Okay, well I went up and picked up the valve today and it was $85.49 with a $25.00 core.

got to my shop, and started trying to get the brake pedal off the truck. Holy cow that was difficult to do!!! The pin that the pedal is attached to the bracket was seized in place. I ended up using a torch and heated up the bracket (catching the truck on fire once, but quickly extinguished with a water hose I had close by) and once it was hot, I drenched the pin with PB blaster and and broke free. I kept working it back and fourth and then started to remove the pin. got it out without breaking the bracket!!

then pulled the plunger and saw that it was in pretty bad shape, so I put it on the wire wheel and cleaned it up then coated it with antiseize. coated the steel pin as well, cleaned out all the dirt around the brake pedal area by blowing air in and around it and re-installed everything.

Tested the pedal and it now feels great!

Luckily the plunger was just at fault for now. I am keeping the Valve on my shelf just in case the valve body decides to take a crap on me.

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haha mine is 36 years old it was ciezed too i had to take every fitting off from underneath and pull it out the top (by that time my hands were missing a lot of skin) then i soaked it in penetrating oil all night torched it then pounded it out made putting the new one in easier with it apart

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