My 20hp. Walker GHS pulls to the left when i'm mowing. I'm always having to tap the right drive lever to keep it going straight. So my stripes are swervy and its harder to mow. I notice it after the machine has been running for a while and the hydros are warm. Someone recommended i check the circumference of the tires to make sure they are equal. If i check the psi will that work or do i need to jack up the mower and measure around the tire? do the hydros need new oil or some type of maintenance? I know once a hydro is blown then its junk and has to be replaced but these don't seem that bad, they just aren't very even and a little rough. Thanks of any help.