Caterpillar doesn't know what to think of this. Seeking some advice... The machine will drive forward just fine. If you try to turn left(right track operating), the brake grabs and eventually locks up due to loosing pressure. It turns right just fine. I have rebuilt and resealed the right hydraulic drive motor. If I tee into the line with a gate valve and a pressure testing gauge, I have 450-475 PSI at the hydraulic motor. I can shut the valve and it will hold the pressure at the hydraulic motor and operate without locking up. I can move the set up to the left hydraulic motor and lock the pressure in the motor seeing 450-475 psi at the left hydraulic motor. The machine will operate just fine. When you let the system flow through lines for the parking brake together without the valve closed the psi at the right hydraulic motor instantly drops to 200 psi and the motor locks up. When the two motors run in unison besides going forward and backward, the pressure bleeds off internally. When you isolate them they work just fine. Does anyone have any ideas of where to go next? I'm not really sure how the brakes on this machine work and what changes and releasesthe pressure allowing the machine to turn. I don't know where to go. If you have a hydraulic routing diagram that would be very helpful, but any knowledgeable advice will be much appreciated. At this point Caterpillar is telling me to buy a $1500 service manual...
Cat can't fix it and they want you to buy a 1500.00 dollar manual? How do they expect you to troubleshoot their design if they can't? Call and see if you can get a schematic of the system, not a connection diagram (although one would be helpful to verify correct plumbing), a schematic will help us troubleshoot more effectively without to much guessing.
Wow. Where to start? Some info on how the system works maybe... First, the two brakes share a common line so whatever pressure you're seeing on one side, the pressure should be the same on the other side. It sounds like you've proved that the brake pistons in the motors aren't leaking internally. There's no hydraulic connection inside the motors between the brake and the rotating group. The next thing I would check is the pressure on the line into the park brake solenoid. If you are sat in the seat, the park brake solenoid is kind of below your left elbow. I think that you'll see this pressure drop off when your fault occurs. What you're measuring is charge pressure. This is supplied by the charge pump which also supplies oil to charge the drive loop. What I would guess is happening is that when you turn, either the RH drive pump or motor has abnormally high internal leakage which is taking all the flow from the charge pump and causing a pressure drop. Was the machine OK before you rebuilt the RH motor? Why did you rebuild it? If it was smoked and you're confident with your rebuild, then the pump probably has some problems. If you only had it apart to reseal it, you might have got something wrong inside it. Unless anyone has any better ideas I think you need to see what sort of flow you're getting from the case drain lines on both the RH pump and motor. You'll have to get the machine off the floor and run it with the cab up. Be careful.
Cmark, I am not familiar with this CAT, but in general most hydrostat systems have a "flushing valve" or "hot oil shuttle" in or on the motors, is that true for this machine and could that be a possible cause of the malfunction?
Yes you're right. There is a loop flushing valve on each motor which could cause a lack of charge pressure. To avoid confusion I didn't go there just yet because; a. It drains through an orifice and if the OP is losing enough flow to cause low charge pressure I was thinking the orifice would probably prevent it. (Although I don't know for sure.) b. The flushing valve discharges through the case drain line so the end result would be the same i.e. a problem with the motor.
Cmark, Gotcha, I like the orificed design, I'm used to the older systems where they just balanced the relief valves to dial in the cooling flow, sort of a "poke and hope" design. One thing I had trouble wrapping my head around with these dual path hydrostats is that both sides are pressurizing even when one is completely stopped, the slower or stopped track is actually holding pressure in reverse mode and thus is just as suspect as the driving side. The fact that this machine only malfunctions in one direction though does seem to point to the right motor or pump.