All told, how much fluid are you going through? We
know the fluid has to be going
somewhere.
Which leads me to the next question; are there any puddles yet? Under the wheel cylinders (wheels)? How about puddles elsewhere? Like
anywhere under your rig. If not. Have a look along, and inside the frame rails (channels(s)). Feel or look along the brake lines path. See if you can find any wet spots, that might indicate a leak. A rusty line only needs a tiny pin hole to be a problem. You might find a puddle inside one of your frame rails, that isn't big enough to leave a puddle under your rig.
Hope this helps.
Chris
