1979 gs1000L petcock problems.

[sschering]

I would think it would run in "Pri," have you tried it? Rebuilding petcocks is a tricky thing. I had three that leaked. One was just an outside O ring under the lever. The second I replaced the diaphram and it still leaked when turned off, I worked the diaphram seat and eventually it stopped leaking, I just don't trust it now, and the 3rd, I reworked extensively, it
still leaks when shut off, badly too, but there's a check valve in the petcock that goes to the the vacuum line and I think it's bad, so no matter what I do I haven't been able to get it to stop leaking.

The 79 Petcock has no prime setting.. Just run and reserve..
When the bowls are dry getting it primed is a big PIA.. suck on the vac line or crank till the cows come home.

That check valve is on the air side of the diaphragm cover.. It's real job is preventing a diaphragm rupture in case of a backfire.
If it's bad and leaking air the petcock will never open.
Fuel out the vac line or check valve vent would be leaking from the diaphragm.

When I did my petcock I just striped it down and threw it in the dip can for 10 min (enviro safe dip from NAPA that won't attack rubber but will swell the crap out of it if left for days). Inspected the surfaces, assembled , tested check valve and gave it a leak test on the tank..

I guess the other odd thing with the 79 is the petcock to tank seal is just an O ring that goes around the outside the bolt holes.
You need gaskets under the screw heads or it will leak there.
The kit didn't come with any so I made my own from a roll of gasket material I had laying around.

[sedelen]

The 79 Petcock has no prime setting.. Just run and reserve..
When the bowls are dry getting it primed is a big PIA.. suck on the vac line or crank till the cows come home.

That check valve is on the air side of the diaphragm cover.. It's real job is preventing a diaphragm rupture in case of a backfire.
If it's bad and leaking air the petcock will never open.
Fuel out the vac line or check valve vent would be leaking from the diaphragm.

When I did my petcock I just striped it down and threw it in the dip can for 10 min (enviro safe dip from NAPA that won't attack rubber but will swell the crap out of it if left for days). Inspected the surfaces, assembled , tested check valve and gave it a leak test on the tank..

I guess the other odd thing with the 79 is the petcock to tank seal is just an O ring that goes around the outside the bolt holes.
You need gaskets under the screw heads or it will leak there.
The kit didn't come with any so I made my own from a roll of gasket material I had laying around.

It's unfortunate that the '79 GS1000L has a petcock that doesn't have a prime position. That makes troubleshooting this petcock harder. I don't believe I've even seen one anywhere for sale. Like you said if the check valve is bad the diaphram which opens from vacuum and allows fuel to the carbs may not open at all. True there's a gasket type of washer under the two 10mm bolts/screws securing the petcock to the tank. The part number for those is 09168-06023 and they're called "washer, oil gallery exhaust" @$1.12 each at Boulevard Suzuki.