I agree with the other coment. If at all possible the panel should be replaced. These panels have several problems with them.
1) The breakers are very unreliable. I have personal expierience with many overloaded circuits (25-30 amps on a 20 amp breaker) that did not trip. I also had a guy cut across a 240v heater wire (that he thought was off) and blew up his wire cutters. He called us out to check out the problem because the breaker did not trip. I found the breaker still on after he had caused a major short circuit!
2) I have many pictures of Zinsco/Sylvania panel damage. The breakers burn up at the connection to the bussing. If anyone would like a copy of some pictures, or copies of some of the personal expieriences with these breakers you can email me at [email protected]
PS: you can add a ground bar (as long as ther is room) and relocate the ground wires to it so you have room in the neutral bar for more neutral wires.
PSS: If you are not confortable with this please call a licensed electrican.
WA State Master Electrician