I have a case 780b backhoe that has the boom stop raising using single lever. If you try to raise the boom little to no motion occurs and perhaps a sight drop in height. When trying to operate it has a hissing type noise which was thought to be the pressure relief valves. both the A and B port relief valves have been removed and all seals replaced and reset. same issue with the boom, all other functions in the valve block work: dipper, swing, and bucket. It has been found that if you slightly move the lever to extend the dipper which is the retracting direction of the single dipper cylinder you can get the boom to raise which is the extending direction of the 2 cylinders. not sure what to try to repair next, hoping not to have to remove the 120lb valve body buried a foot inside the frame if at all possible. thanks for the advice in advance.
Welcome to HEF @Scott Sanicki You will need to inspect shuttle valve located in the rear of the boom section - in fact reseal all the shuttle valves for all the sections - if one seal is gone they all will be in similar condition. When you remove the plug there is a small ball bearing #18 that will come out - don't lose it...... I'm not sure exactly how the valve is positioned on the 780b & if you can get access to them very easily. Oring #17 will likely be missing & is a usually a white material which is harder than 90 duro - if you fit a normal oring, they won't last long. The 780b parts listing does show a 90 duro oring - however the crawler/dozers use this same valve & they use the white oring only found in an R30185 seal kit (not cheap)
Do the primary shuttles also.
thanks gents looks like I will have to drain the system and do the dreaded job of pulling out the 120lb control. Will let you know how that fix works.
per suggestion took out the block and pulled the shuttle's and you we correct the white seal were incinerated for most of the sections and a few other seals here and there. back together again and it is working better than I bought it years ago. thanks for the help gents!
how I should have stated for others most of the finds where on the primary shuttles item seal number 13. also I would say by extra seal 14 as they are small and hard to get over the tip I split a few installing so I had to repurpose some old one that where ik, hope that doesn't bite me later.
Glad everything worked out for the good.