I have a Bobcat 7753 that keeps blowing the right drive motor…

Well, ya really threw a curve ball at me, haven't worked on one that doesn't have case drain filters, had to go find an old book about one. Sure enough, early s/n machines didn't use case drain filters. However, according to the flow charts, the case drain lines are the same on the different models, maybe you should consider adding them to your machine as they provide a good way to quickly check the internal condition of the motors.

Back to the problem, the flow chart shows the right drive motor case drain going back to the case of the drive pump, which also gets charged by the charge pump. The left drive motor case drain returns to tank. This might explain why it blows the right seal but not the left seal. Now just gotta figure out what's causing the excessive pressure inside the case of the motor to blow the seal as I'm certain that's what is happening. Again, this is typically caused by a failing motor, but if your certain your motor is good the pressure is coming from somewhere else.

I thought about switching the left and right drive motor case drain lines, make the left motor go to the pump and the right motor go to the tank, but the result might possibly be now it would blow the left carrier seal, but I don't know that would be the desired result, another motor to pull and replace a seal, yuk.

I'm still noodling through this diagram to see if I can figure out another method to work through this problem and report back, I wanted to reply to let you know I haven't forgot about it, just still scratching my head. I'm replying with "need info" as I haven't provided an answer thus far.

Were you able to view that diagram I posted? I haven't had to tear into the drive motors on a 7753 so I'm not real familiar with them, but one thing I'm curious about is that check valve (item 9) on each motor. Maybe it's nothing, but a check valve serves a number of purposes, two of which would be 1) prevent flow from going in a certain direction, and 2) prevent pressure from going in a certain direction.

If you look at the diagram, it shows 2 case drain lines connected to each motor. One is a straight drain line to the motor, the other goes through that check valve. I'm not sure what function the check valve serves, again, flow charts don't show us that as schematics would. But I'm wondering what would happen if that check valve failed and allowed flow/pressure back in that direction, just a thought, it might be worth a look and inspect those check valves.

Aside from that, by looking at the chart, if that motor is getting excessive case pressure it would have to come from internal motor wear or too much pressure inside the case of the pump. In a simple Bobcat circuit (minus a bunch of accessories) oil goes from the gear pump to feed the main loader control valve. Oil leaves the control valve and goes through the oil cooler and the hydraulic filter. Then the oil goes to the drive pumps to charge the pumps. Excessive oil the drive pump case goes back to the gear pump to repeat the process. Fairly simple circuit. And we see the only difference in the two drive motors is the case drain connections, left motor drains to tank, right motor drains to drive pumps, hence my thinking, you either have a faulty check valve at the right motor, right motor is faulty causing too much internal case pressure, or there's something amiss inside the drive pumps that's causing back pressure on the right drive motor blowing out the carrier seal. There no way I could prove which is causing the problem.

So, where to go. One thought I had was say to heck with that case drain connection to the pump and send it to the tank like the left motor does, but I don't know if that would adversely affect the drive pump. I don't think it would, but I can't say for certain. Another thought I had was tee into the motor case drain lines with a 200 psi gauge and monitor pressure. The charge pressure for the drive pumps should be 155 - 165 PSI. If you're getting that kind of pressure or more on the right motor case drain line that might be your problem, too much pressure from the drive pump charge. Again, maybe we should send that line to the reservoir to prevent this.