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low-press. return line blowing out on belt-drive system

Discussion in 'Fisher Engineering Discussion' started by Megunticook, Jan 9, 2016.

  1. Megunticook

    Megunticook Senior Member
    from Maine
    Messages: 190

    Have an eighties-era Fisher belt-drive system on my truck, fully restored by me and has always worked great. Today I flushed the system (do this every year) and changed the inline canister filter on the low-pressure return line (from the valve body to the pump reservoir). I also emptied the old fluid from the angle rams and replaced with fresh.

    After making sure the pump reservoir was full, I set up the plow so the carry chain was holding its weight, collapsed the lift cylinder fully, and cycled the plow blade side-to-side to bleed the system. Then I topped off the reservoir. Everything seemed fine until the plow started moving very slowly, I shut her down and found the return hose had leaked almost a quart of ATF from the end of the hose where it fits over the quill barb fitting at the valve body.

    I swapped in new a new hose between the valve body and the filter, refilled the reservoir, did the same procedure and the filter blew off the hose I had just replaced (I had tightened a hose clamp around it there).

    Thinking the filter might be blocked, I changed the filter and tried it again. Seemed fine, but the hose blew out again at the filter.

    What in the world is causing that line to get so pressurized? It's supposed to be a low-pressure return line.

    Could it have something to do with an air bubble in the system? Or some sort of blockage downstream of the filter?

    I'm totally scratching my head here. Any help appreciated.


    CARDOCTOR PlowSite.com Addict
    Messages: 1,304

    can you remove the filter maybe put a piece of steel tubing in its place to see if it makes a difference?
  3. Megunticook

    Megunticook Senior Member
    from Maine
    Messages: 190

    I swapped in another new filter (first checking to see that it wasn't blocked) so figured I had eliminated the filter as the issue.

    I think what happened was that I had replaced the cap on the pump reservoir tightly, which prevented any air from escaping and perhaps built up pressure in the return line as the valve body sent fluid (and air bubbles) back to the reservoir. Eventually the air pressure became so great it blew the filter right off the hose.

    This afternoon I tried loosening the cap on the reservoir before going through the bleeding procedure. No blowout this time, and when I stopped I noticed the fluid level was way down in the reservoir and the fluid looked foamy. I'm guessing that loosening the cap allowed air to be purged out of the system. In any case I topped it off again and will do another test tomorrow--big storm coming tomorrow night!

    In the future I'll make sure the reservoir cap is cracked loose before doing my bleeding.

    Well, look on the bright side--at least I gave my system a good flush (lost a good 2 quarts between the three separate blowouts). Although I had just finished flushing the system earlier this week. As long as I didn't damage the pump by cavitating it (it sounded and performed fine this afternoon), I think I'm good. It did run for a minute or two with the reservoir essentially empty after the first blowout because I didn't see it right away.

    Thanks for the help.
  4. Megunticook

    Megunticook Senior Member
    from Maine
    Messages: 190

    Crap--it happened for the fourth time this afternoon. This time it created a little engine compartment fire which was a lot of fun to deal with. Haven't seen one of those in 25 years. Fortunately it didn't do too much damage although when I first opened the hood to see what was going on I saw a whole lot of flame (slammed the hood shut, grabbed an extinguisher, and by the time I carefully opened the hood again the fire was mostly out. Burned right through a couple of ignition wires though).

    So today I again bled the system with the cap off (blade suspended by carry chain, lift cylinder fully collapsed, angle back and forth all the way and let the relief valve open up for several seconds on both sides, raise and lower blade fully, stop, top off fluid, repeat, etc. Probably added another pint or so.

    Drove a short distance to pick up some wood (with reservoir cap on but loose). Dropped blade, shut off motor, and that's when I saw the smoke. When I dropped the blade, ATF had apparently squirted out the end of the return line where it fits on the valve body.

    Why is that line getting so pressurized? When the lift cylinder collapses from the weight of the plow blade I get how that would create some pressure, but isn't that normal operation? Is something constricting the return line? Could the valve body somehow be malfunctioning and delivering some of the pump pressure to the return line?

    I guess my next move is change the hose on the return line downstream from the filter. I have an extra valve body, I guess that would be the next choice.

    But after that fire, I'm very anxious to get this fixed.

    Funny, the plow has been bulletproof since I fixed it up in '08, I wish I could figure out what the issue is!
    Last edited: Jan 12, 2016
  5. Megunticook

    Megunticook Senior Member
    from Maine
    Messages: 190

    The more I think about this, the more I think it's the S.A.M. aftermarket filter. It all started when I changed the filter--and the leaks/blowouts are all upstream of the filter. Normally I use OEM Fisher filters.

    I'm guessing whoever the Chinese supplier is for the S.A.M. filter put the wrong filter element in there, and it isn't porous enough to allow ATF to pass through (maybe they used a filter element designed for a more viscous liquid like gasoline). In other words the design is wrong, and S.A.M. didn't catch it.

    Seem reasonable?
  6. As someone said early on in your thread, take the filter out and put in a union or piece of steel tubing and see if the problem is resolved.