I have a shindaiwa t27 trimmer that i've been trying to get going. I replaced the piston rings, and diaphram in carb. It will start and run for just a few seconds on choke then die. I noticed today that if I hold in on the brass overflow valve(20011-81550) in, it runs perfect. I can't find a close up pic of the overflow valve, but it looks like it has a couple of grooves for o-rings. So my question is, does the overflow valve require o-rings. Thanks for any help.