I agree, keeping the ports clean helps. Also, checking the guide track for debris is important. That's how I figured out my 2nd valve issue. It was really hard to see with out a light.
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Thanks guys for the replies.
I cleaned out the horn with the QHR (Quick Horn Rinse + suddser). I did it outside as it's still 68? outside :o. I placed the horn on a white towel so I could see if any debris came out. Couldn't really see anything, but who knows.
After I posted above, I took out the 3rd valve and saw how the guide comes out. Yep, you have to work it out slowly (worry it?) up and down. I used the the valve stem to help lift it until I could grab it with finger nail. I was going to swap the 3rd guide with the 4th guide to see if that was the problem (they are both the same size), but when I held them up to the light in the kitchen, I noticed the 4th guide wasn't square. The bottom of that guide was sticking out more - or would go into the groove deeper. So I carefully filed it square with a metal nail file.
After drying the horn, lubing the slides and oiling the valves it seems to be fine now. I understand it may take some time to 'break in' or 'seat in' the valves. I think I was spoiled with the 3 valves working flawlessly since I got it.
I suspect you are very careful with your horn, but I have often seen this when the valve is inserted and the guide is allowed to strike the top of the casing. It disabled Ann Baldwin-Shreves' Willson just before a performance. So I took out my trusty, tiny Swiss Army Knife and used its nail file to do just when you did.