I was riding a ZRX400 last week with knackered head-bearings and it rode fine.. however when going over bumps and drops the was about 1/2 inch of play in the stem and it felt like the front was going to drop off
Anyhow..
do not ride with a leaking forkseal!! besides MOT failure if your pulled up for a spot check you'll be in trouble but the worst part is (and this happened to me) was I was riding a bike with a knackered fork seal and though i'll change this the weekend, the leak got a lot bigger the one ride and coated the tyre, brake pads and disk in fork oil... come to slow down and lean in for a turn and it didnt' slow very good and the bike low-sided down the road... :/ (it also ruined the pads!)
Just check the obvious. Start with tyre pressures first (they are inflated right?)
Headbearings.. these usually give a notchy feel if worn or play if loose... push the bike down with the front brake on and see you can can feel any movement in the head bearings either forward / backwards or up / down. The bars should turn left / right freely and smoothly.
With leaking forks your going to have an imbalanced suspension so this will case the one side to compress more than the other, which will obviouly affect the bike so get that sorted asap.
Check the front wheel bearings whiel your at it.. lift the front up and grab the wheel at 9pm / 3pm position and rock the wheel left and right, there should be no play, do the same at 12/6pm with an up/down motion. again there should no play and the wheel should spin freely.
Also check none of the brake pads are binding on the disks.
Quick Recap.
1. Sort fork seal first, do both at the same time and refil both with correct fresh fluid, make sure rebound / dampening / compression etc are all set up correctly for your weight.
2. Check headstock and wheel bearings.
3. Check for binding.