How to bleed air from hydraulic system? - CAT 304

Hi all,

I'm starting a new thread for this issue. The original issue resulted in me removing the main control valve, cleaning it up, and installing new seals: https://www.heavyequipmentforums.co...one-side-bogging-down-engine-cat-304cr.95940/

It's a 2004 Cat 304cr (NAD00987).

Now that everything is back together, I can't seem to get the air out of the hydraulic system. All of the controls are weak. The blade and the boom do not have enough power to lift the machine. It'll swing alright to the left, but will barely turn to the right.

From what I've read, the air is supposed to automatically bleed back into the tank. I've tried cracking the plug on the top of the pump and oil comes out. I can't find anything else in the service manual that addresses this issue.

There is a section of the service manual that mentions raising the boom, shutting the engine off, and lowering the boom with the engine off to pressurize the system. I followed the instructions but my hydraulics do not function unless the engine is running.

In past situations like this on other machines, I've been able to slowly move the cylinders in and out, and eventually, it clears up. I'm not sure why this machine is so stubborn. Should I start cracking some hoses near the cylinders to see if I can get the air out?

Howey Matt said:

Now that everything is back together, I can't seem to get the air out of the hydraulic system. All of the controls are weak.

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My opinion. I think that air in the system may be a bit of an assumption on your part.
You had a totally trashed final drive which I assume will have spread shrapnel right throughout the hydraulic system. Unless you undertook a thorough cleaning process of the whole system there is every chance that your problem could be caused by one or more particles from the failure lodged where they shouldn't be.

Nige said:

My opinion. I think that air in the system may be a bit of an assumption on your part.
You had a totally trashed final drive which I assume will have spread shrapnel right throughout the hydraulic system. Unless you undertook a thorough cleaning process of the whole system there is every chance that your problem could be caused by one or more particles from the failure lodged where they shouldn't be.

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I wouldn't describe my cleaning process as thorough, so you're probably right. It has to be something that is affecting the entire machine since all of the functions seem to be affected. Maybe I can test some pressures and get an idea of what's going on. Too bad the main control valve leaked the first time I reinstalled it - everything was fine then. Must be the new seals!

I’ve been messing around with it. Pressures are definitely low...

I’m getting 2,000 psi each for the pressure relief bucket (P1) and the pressure relief stick out (P2). They’re supposed to be almost 4,000 psi I believe.

Pressure relief boom swing left is 1,500 psi, but swing right is only 400 psi. They're supposed to be 3,000 psi.

Pilot pressure is around 400 psi. Supposed to be 495 psi I think, so just a little low.

I guess the question is what would cause pressures to be low on all circuits? And why is psi for swing left and swing right so different?

I think I'm going to pull the main control valve for a 3rd time, clean it, and reinstall. That's the only thing that changed. What's the best way to clean it thoroughly? I just used compressed air the last two times.