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Air Gun Home Forum Index » Rifles » QB78 Valve pressure relief    View previous topic :: View next topic  QB78 Valve pressure relief   Posted: Tue Oct 04, 2011 10:05 am SMP Silver Status Member Joined: 04 Feb 2011 Posts: 1183 Location: Just south of "Out There"- Land O Lakes Ontario Is there a pressure relief on a QB78 valve? I'm trying to run mine on HPA and everytime I get to 800psi it vents out the bottom of the tube. It holds once it drops to 800. I'm shooting for 1000-1100 psi Or possibly the valve assembly isn't screwed tightly together? _________________ Sean   Posted: Tue Oct 04, 2011 11:27 am Alstone Moderator & Site Supporter Joined: 01 Mar 2007 Posts: 4139 Location: Linconshire, ENGLAND There's no pressure relief valve on a QB, you probably have a faulty seal etc. Al _________________   Posted: Tue Oct 04, 2011 12:13 pm SMP Silver Status Member Joined: 04 Feb 2011 Posts: 1183 Location: Just south of "Out There"- Land O Lakes Ontario Agreed. It's in bits now. there are no blemishes on the seal but that's the only possible cause. _________________ Sean   Posted: Tue Oct 04, 2011 1:49 pm Alstone Moderator & Site Supporter Joined: 01 Mar 2007 Posts: 4139 Location: Linconshire, ENGLAND When you say that air is escaping from the bottom of the tube, where exactly do you mean? Just another thought, do you put the valve into the tube before tightening it up, I know it a daft question but worth asking . Al _________________   Posted: Tue Oct 04, 2011 3:59 pm SMP Silver Status Member Joined: 04 Feb 2011 Posts: 1183 Location: Just south of "Out There"- Land O Lakes Ontario When I wrote that originally the gun was assembled and it was generalized to "bottom of the tube' Now that I have stripped it down it is coming out of the valve retaining screw and thats equally odd. if it was just a leaking valve o ring it would come out everywhere. not just the screw hole. I think I will give it the old soapy water bubble test just in case there is a hole/crack leading into that screw _________________ Sean   Posted: Tue Oct 04, 2011 4:19 pm rsterne Moderator Joined: 15 Jan 2008 Posts: 2998 Location: Coalmont, BC Sounds like the screw was overtightened and the valve body is cracked.... Either that or the hole was drilled too deep.... Blue Locktite or even the purple hydraulic sealant as a repair?.... Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Airsonal: Too many to count!   Posted: Tue Oct 04, 2011 4:25 pm SMP Silver Status Member Joined: 04 Feb 2011 Posts: 1183 Location: Just south of "Out There"- Land O Lakes Ontario well it passed the bubble test at 125psi. But that certainly doesn't mean there isn't a crack there thats opening up a higher pressures. I was thinking the same thing myself Bob. A drip of blue is my only choice here at home. That or Gorilla brand crazy glue. I'm just waiting for a trip up to Eric's at this point for a seal kit. _________________ Sean   Posted: Tue Oct 04, 2011 4:28 pm Alstone Moderator & Site Supporter Joined: 01 Mar 2007 Posts: 4139 Location: Linconshire, ENGLAND The valve retaining screw is on the low pressure side of the valve if it is leaking there then it probably is the valve seat or seal? Al _________________   Posted: Tue Oct 04, 2011 4:37 pm rsterne Moderator Joined: 15 Jan 2008 Posts: 2998 Location: Coalmont, BC Quote: The valve retaining screw is on the low pressure side of the valve if it is leaking there then it probably is the valve seat or seal? DUH!.... Yes, of course.... and if it's leaking there, it would be leaking out the barrel/breech.... So we're back to the O-ring if it's not leaking out the barrel with the bolt closed.... Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Airsonal: Too many to count!   Posted: Tue Oct 04, 2011 7:15 pm SMP Silver Status Member Joined: 04 Feb 2011 Posts: 1183 Location: Just south of "Out There"- Land O Lakes Ontario Eureka!!! As I was fiddling with the valve cleaning it and removing burrs I screwed it together and noticed that it came tight with a considerable gap. I measured everything and found that the Oring was only being squeezed 0.001 -0.002" After a little more investigation I found that this sleeve causes the valve to come tight before any pressure is applied to the Oring so I shaved it 0.010". After a quick shave and deburr I reassembled the valve into the tube and pressurized the tube.....No Leaks! I'm leaving it to sit overnight at 1500psi while I go watch some tele with the Mrs and polish the hammer parts. On a good not it gave me a reason to clean and deburr the tube _________________ Sean QB78 Valve pressure relief     Air Gun Home Forum Index » Rifles You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum All times are GMT - 5 Hours   Page 1 of 1   All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First  Select a forum Main Forums----------------AirGunHome Q&AGeneral Air Gun Questions and TopicsPicture GalleryTarget Shooting: Casual, Fun and CompetitionHunting, Pest Control, Varminting People/Events/Airgun Issue Discussion----------------Introduce YourselfThe BS TopicClubs/Events/Regional/Political Airgun Issues Maintenance, Modification and Reviews----------------RiflesPistolsAntique, Collector and Novelty AirgunsBB GunsPellets/AmmoOpticsAccessoriesAirgun Smithing Buy, Sell or Trade Forums----------------For SaleWant to Buy   Note: If you are seeing "Please enter your username and password to log in." 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