Any pressure cleaner wizards?

I have a 5hp electric pressure washer that uses a CAT 3DX pump. The pressure has been falling off and now is at a point that it is virtually unusable. The input filter is clean, I took the unloader apart and things look OK in there. I am now left with the pump itself I think. I pulled it apart (the 8 allens and the 3 caps). The pistons look OK, looking in the holes, I don't see anything obvious with the check valves, no broken springs, no contamination. I can see all the way through all the passages and they are open. I think I am down to ordering a rebuild kit but by the time I get a seal kit ($90) and a check valve kit ($164) I am 85% into the price of a hole new pump assembly with everything attached.
This is a $700 pressure washer, the pump is $300 and the kits are $254 plus shipping.
At this point I am about to start pumping WD40 or some other solvent through it to see if that doesn't free something up. I don't see I have much to lose since tossing the pump is the next step.
I would really enjoy hearing something better since I have another CAT 4 piston pump on the shelf that is bad the same way and a little electric pressure washer that doesn't work that well either.
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Thanks but that was the first thing we did.
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I am no expert but i have rebuilt many of those style piston pump heads for a spray car wash business.
Similar symptoms as you describe. The sprayers would go from great to suddenly loosing pressure. After another week of use the sprayer would be practically useless.
90% of the time a rebuild of the pump head would fix it. Some times the metal check valves at the top of the piston would be the culprit other time the cups or seals in the piston itself would be the problem. Eventually we would just rebuilt the whole top. Most times there would appear to be nothing obviously worn on the parts so even the smallest amount of wear must cause the trouble. I did notice over time that the metal check valve cups would appear polished where the new ones had a dull faced flat finish.
If your sprayer is well used then it could very well be the piston or check valve assembly. If it has been accidentally run dry on occasion that will speed up the demise.
Occasionally, when our stock of pump rebuild parts was depleted i experimented with trying to refinish the metal check valve cups and their seats by honing or lapping with various fine finish files , stones and emery cloth to get a pump working again for a while. I would essentially take the metal cup and hone or lapp against a known flat surface and try to get the finish to the similar satin finish the new ones would have. Doing this made it obvious that the polished wear rings were actually uneven , and deeper than apparent to the naked eye. I suppose if the valve turns a bit the uneven wear will cause them to tilt and zap the pressure.
anyways I hope this helps.
robb
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Thanks Robb. That is what I was believing myself. We have used this a lot and our water really sucks. I am going to try the WD40 thing, just to be sure it isn't just gummed up somewhere but I think a new pump is how I am going to go. The parts are almost as much as a pump.
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On Thu, 22 Sep 2011 14:48:40 -0400, snipped-for-privacy@aol.com wrote:

OK I did the WD40 trick. I attached a hose to the intake and one to the output, and cycled about a pint of WD40 through there until it got up to about 110 degrees, (can getting hot in my hand, a minute or so). I turned it off, hooked it up and I got 2600 pounds when the unloader popped off and about 2100 with a 15 degree nozzle on it. I can live with that.
One more thing WD40 can do for you.
I still think a pump is in my future but not right now.
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