Tractor Repair: 3930 hydraulics 1991 4x4 pump in tranny syncro, brush hog, hook ups

Question
my hydraulics are weak after 15 minutes or it comes to normal running temp. i pulled the top off and reserved lift cylinder and replaced all O rings and any other parts worn inside because it leaked off soon as it was shut down, thought maybe it was bypassing some where which lift cylinder looked like some one beat on it with ball pean hammer where lifting rod had banged on cylinder wall put 1250.00 in parts  i am a mechanic by trade, them i thought maybe hydraulic oil was to thin so i used ISO 68 instead ISO 42 did not help i know it sounds like pump is weak it will lift after warmed up only if i run up rpm to 1500 the brush hog is heavy 306 heavy duty brush hog brand i would like another opinion on it i have a small 25.00 parts book but not quite sure where to hook up gage to check pressure on it. can it be done from remote hook ups or off of control valve body i hate to put a pump in it if that not the problem it is 900.00 for pump any info would be greatly appreciated in book it says to check pump preassure at 1650 rpms and it should be 2650 psi is the hog to heavy to pick it up at idle i would not think so 3058 max lift capibility thanks

Answer
william black Category: Tractor Repair Private: No
 Subject: 3930 hydraulics 1991 4x4 pump in tranny syncro Question: my hydraulics are weak after 15 minutes or it comes to normal running temp. i pulled the top off and reserved lift cylinder and replaced all O rings and any other parts worn inside because it leaked off soon as it was shut down, thought maybe it was bypassing some where which lift cylinder looked like some one beat on it with ball pean hammer where lifting rod had banged on cylinder wall put 1250.00 in parts  i am a mechanic by trade, them i thought maybe hydraulic oil was to thin so i used ISO 68 instead ISO 42 did not help i know it sounds like pump is weak it will lift after warmed up only if i run up rpm to 1500 the brush hog is heavy 306 heavy duty brush hog brand i would like another opinion on it i have a small 25.00 parts book but not quite sure where to hook up gage to check pressure on it. can it be done from remote hook ups or off of control valve body i hate to put a pump in it if that not the problem it is 900.00 for pump any info would be greatly appreciated in book it says to check pump preassure at 1650 rpms and it should be 2650 psi is the hog to heavy to pick it up at idle i would not think so 3058 max lift capibility thanks.

Hi William
You seem to be on the right track replacing the parts that you did. But do not like the sound of the damage to cylinder wall !! Nothing should touch this but the piston of course. There might be some resistance in the system as you are pushing the revs up before any thing happens. You do not say if you checked the filters they may be blocked!
1991 is not that old for hydraulics the usually have a long life.
But on the other hand if there is damage to the hyd cylinder oil could be leaking past the piston and taking a short cut down the bore! Thus loosing pressure. And not doing the lift that it should ! if your engine had a damaged cylinder or 2 you would soon loose power at the wheels –right !
You should get a good manual this will show you were to connect an external pressure gauge !
Martin