Have a Cat 426 Backhoe(will include pin, serial, etc. in pictures below), that lost Boom operation. Was running fine for about 20 minutes, then slowly got worse for about 5 minutes, then no movement out of the two boom cylinders. Let it cool down overnight, and starting it up cold it will lift the two boom cylinders about quarter to half way up. Also, I do not know if it did this before, but the boom will leak all the way down from about halfway retracted to the ground within 24-48 hours. Looked in the manual and it stated that if only one hoe function is affected and not multiple(All the other ones work), plus if you stall a hoe function before that valve body, in my case the only one before the boom is the left outrigger, and the function(boom) works fine then most likely it is the primary resolver valve itself or the o-rings on the primary resolver. So I stalled the left outrigger and the boom works perfectly, without stalling the outrigger it does not. So I replaced the Primary resolver for the Boom cylinders, the old one had cut orings on bottom and forwards the top it had about half the o-ring left, I was able to get all of it out I believe, which was just a little bit still left in the valve body. It still does not work, so next I’m looking at the secondary resolvers on it. The manual said if it was secondary resolver problems that more than one function would be affected. But only the boom function is affected. I changed out the boom and left outrigger primary resolvers out with the new updated ones with Teflon backup orings, and the other primary next to the boom I just replaced the o-rings on it. Still nothing although I didn’t expect those ones to do anything only the boom one. Anyone have any suggestions on what to look for or what to check? I’m going to be pulling and resealing the secondary resolvers Tommorow on the boom and the two next to it.
Cylinder drift is usually measured over a 5-minute time span. If the cylinders don't move apprecibly in that time it would suggest they are not bypassing internally.
Good to know, One important thing I left out it is only the Boom Up that is weak/not working. Boom down is working fine and has plenty of power to lift the machine.
I reckon you could still have a (secondary) resolver problem, not just the seals on the body. The resolvers are all the same Part Number IIRC. I would suggest to try moving the secondary one from the boom to another location and see what happens. Also closely inspect the condition of the ball #21.
I have new seals and the #21 Ball for the secondary’s already, so I’ll take two out switch them around reseal and put in a new ball in each. That will happen later today, I’ll update once it happens.
Ok, pulled the boom’s secondary resolver and found a cut inner o-ring, same with the resolver I pulled next to it, the ball on each were perfect, Resealed and put new a ball in each, Boom is working now.