NOS T250/T350/T500 petcock with slight leak

[jabcb]

A few years ago I bought two NOS petcocks for my parts inventory — was a bit before the price went up to ridiculous $$$$.
Recently installed one on a nice survivor T250, but it had a slow drip leak.
Then used the other one, which is fine.

Took the leaker petcock apart & everything looks good.
The petcock has a small vent for the diaphragm that was plugged up with something that looked like dried varnish.

I cleared out the vent, stretched the spring a smidgen, and reassembled the petcock.
Tested the vacuum chamber with a hand-operated vacuum pump from HarborFreight — it holds 5 in-hg vacuum.

So I have two questions:
1) does it make sense the the plugged vent hole would cause a slow leak?
2) how much of a vacuum should I use to test to make sure that the petcock opens? (Didn’t stretch the spring too much.)

[GTandcbr]

I think the purpose of the vent is to allow the diaphragm to return after being sucked out so if it was blocked it would prevent the plunger and o ring from returning so allowing the fuel to drip. The amount of vacuum required i think is very small to operate the tap. I test mine by putting the tank on the bench with the tap overhanging the edge with a container underneath. Stick the vacuum pipe in your mouth and gently suck. The fuel should flow when you suck and stop when you dont.

[jabcb]

That’s what I was hoping. So then clearing the vent should have fixed the small leak.

I also took an old petcock apart. I was surprised at how horrible the diaphragm looked.
The petcock itself cleaned up nicely so I ordered a new diaphragm from the German eBay seller.

[jabcb]

Rats, clearing the vent on the NOS petcock slowed the drip down quite a bit but didn’t stop it.

[GTandcbr]

Couple of things you can do here. You can change the O ring on the end of the plunger just make sure its petrol proof. Test by dropping it overnight in a glass of petrol. Secondly if you have a dremmil, using a fine detail mop just gently go up and down the tube the plunger fits into to make it smooth and shiny. Not too much mind we dont want to make it any bigger. I then put a smear of silicon grease over the O ring and that sorted it.

[jabcb]

The O-Ring store has a good select of viton o-rings: http://www.theoringstore.com
Does anyone know the correct o-ring size? Or perhaps a size that works better than stock?

I’ll take a look at the sealing surface.
A Dremel in my amateur/home-mechanic hands might be a bit risky.