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The other possibility is its leaking into the airpack. For a few extra dollars if you havent already buy the brake fluid reservoir so you can eyeball your level without having to pop the cap all the time .I found mine on ebay .thanks for the reply. 'have had to halt work on the M 813 brakes for awhile . waiting for parts to replace water pump and on arrival will put on new power steering pump; new radiator;new water pump then start the truck to see where I am with the brakes. I have cleaned the entire brake system taking all of the lines loose from the wheel cylinders blowing air through the system purging all old fluid. installed new wheel cylinders on all wheels and reconnected the lines. Filled the system with DOT 5.