Hi everyone,

I have a problem with oil overflowing out of the filler cap of my hydraulic system's tank. The system is open centre with a diesel powered pump.

When idling, the system is fine. When the valves are engaged (to run two motors) the oil leaks spuriously from the filler cap for a few minutes (quite foamy).

Any ideas on what could be causing this?

Your fluid level is too high and your return line termination is above the fluid level or no return tube at all...

It definitely could be the things that Maglub mentioned, but I would check the suction line connection from that tank all the way to the pump. I have seen this happen a few times. If air enters your system through the suction, the result will be foaming oil, should be noisy too. While your checking the suction lines over, see whether or not you have a suction strainer, and if you do, clean it or remove it if there are no rocks bolts or animals inside your tank.
Also check the condition of the shaft seal, poor a little oil if you can on the shaft seal while it's running if you can, an listen for a change in sound. If the sound changes, your pump has been pulling air in past the seal (actually atmospheric pressure pushed it in, but whatever, you know what I mean). If you have water in your oil, this reduces the oils ability to de-aerate.

i agree with Doug and Maglub, air in in a hydraulic system causes overflow in the tank, but where is this air coming in ?

Well, depending on the pump you have in your system air can come in thru the shaft seal, If the return line is above the oil level and if the tank is too small for the flow your system is handling.
There is another one, if the return line is too small for the flow (flow speed too high).

Regards.

Kamil Rhu
Hydraulic Calculations, videos, articles