Promaster 300 deck wont raise

[tekhead2004]

Hey all - I'll try and give the full picture here. I bought a promaster 300 a few weeks ago. When I bought it, raising the deck was sluggish. I changed the oil and filter in the hydro system (used SAE 30 non detergent general purpose oil). After I changed it, I started the mower and the deck was going up and down fine. Since then, I am getting bubbles in the tank sometimes bubbling out of the top, and now the deck wont go up. It is almost like I have air in the system. Any idea the best way to bleed this system?

Model: 990002
Serial: 002051

Thanks!