tonyishuk Posted March 11, 2019 Share Posted March 11, 2019 Had a horror moment when I saw a rear flat tyre on the mhome at home. Tyre pressures checked three weeks ago. 70 psi down to 10psi After investigation ,it looks as if I have a leaking tyre valve. For some reason the valve had lost it's cap, maybe the pressure had built up an forced the cap off. Replaced the cap and pressure is holding better. Until now I thought of the cap being just that and having no sealing properties. I am replacing the insert and hoping there is no muck in the valve otherwise it's a metal valve replacement. Am I correct in the assumption that Schrader cores are standard ? Are some caps better than others ? Although I would have little faith in using them as a seal. Maybe a lesson learnt to check for air leaks before replacing the caps? Something I will be doing in future. Rgds Link to comment Share on other sites More sharing options...
Keithl Posted March 11, 2019 Share Posted March 11, 2019 Tony, Schrader valves don't normally lose pressure and certainly don't blow the dust caps off! (Well at least not in my experience!). It sounds more likely that some unknown person has let your tyre down! Blow it back up and keep an eye on it. Keith. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted March 11, 2019 Share Posted March 11, 2019 Tony As you’ll see from the following link there is a wide variety of Schrader valve cores https://tinyurl.com/yy2abq2t However, there are only a few ‘tyre valve’ (Standard Air) cores and (based on the following advert) http://www.tyre-equipment.co.uk/acatalog/Schrader-Valve-Core-085000020-085000020.html the Schrader ‘norm’ in the UK may be the 085000020 core. I had a feeling that there were ‘long’ and ’short’ Schrader valve cores, but that may well be wrong. But if you replace the core in your existing valve with a look-alike that is a genuine Schrader product, you should be OK. Regarding valve ‘dust caps', Schrader clamp-in tyre valves used to come with metal screw-on caps that are pretty much indestructible. However, they now come with tough plastic caps that avoid the possibility of a metal cap corroding on to the valve’s metal stem. A properly fitted undamaged valve cap won’t blow off the valve stem, but a poor quality/split plastic cap could. The dust cap prevents dirt from entering the valve and damaging it. It also forms a pressure seal, helping to prevent deflation of the tyre due to slight air seepage past the tyre valve. Years ago it was discovered that, when vehicles began to be capable of very high speeds, centrifugal force would depress the valve core and deflate the tyre and, if this happened, a valve cap would stop air being lost. When you obtain a replacement core, ask the provider (presumably a local tyre-fitting company) for a Schrader cap. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted March 12, 2019 Share Posted March 12, 2019 Apparently I was correct in thinking that Schrader tyre-valve cores could be short or long, as the attached photo shows. Although still marketed, I think the long external-spring core was the original design and the short core (with the spring inside) is the norm nowadays. My understanding is that both types will fit into the standard Schrader tyre-valve’s stem. Link to comment Share on other sites More sharing options...
tonyishuk Posted March 13, 2019 Author Share Posted March 13, 2019 Thanks for your replies. I hang my head in shame! As luck would have, I found the cap on the ground alongside the mhome, I probably knocked it under the mhome when checking the tyres a couple of weeks ago and did not check / replace the cap. IIRcall, it was a weather day! The mhome is parked in a fairly secure area, so unlikely to be deflated by persons unknown, although possible. Taken the core out and nearly lost because it's very small, about half the size of the newer smaller core shown in Derek's photo. Replaced and inflated, still bubbles on the "spit and finger test" but not as bad and holds pressure with the cap on, no bubbles after a minute or so. Mhome goes in for MOT and service shortly, so will get the garage to take a look. I would imagine the problem was there after new tyres and metal valves were fitted a couple of years ago, but not realised until uncapped. Rgds Link to comment Share on other sites More sharing options...
Derek Uzzell Posted March 14, 2019 Share Posted March 14, 2019 The left valve-core in my photo above is commonly referred to as a “short core” (and it’s clearly a lot shorter than the one on the right) but - as you’ve found out - there is also an even shorter version as pictured in the attached photo below. As you’ll see from this advert http://index.weldtite.co.uk/products/detail/short-valve-cores the ‘ultra-short’ core is also generally referred to as a ’short’ core. I’ve seen discarded ’short’ and ‘ultra short’ valve cores lying around on the floor of tyre-fitting outlets, but I’ve never bothered to explore what the significant technical differences are. This USA wepage comments on tyre valves, cores and caps, https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=208 Link to comment Share on other sites More sharing options...
steveandlisa Posted March 16, 2019 Share Posted March 16, 2019 Plastic tyre valve dust caps are just that! their job is to help prevent dirt and dust getting into the valve itself and interfering with the valve seals which could cause pressure loss. They are not designed specifically to hold back air pressure. However there are metal dust caps which have a rubber seal inside and can form a secondary pressure seal, but the seal can wear over time with continuous use. Steve Link to comment Share on other sites More sharing options...
Derek Uzzell Posted March 17, 2019 Share Posted March 17, 2019 It depends on the ‘plastic’ cap’s design and material... This link is to the Schrader catalogue and Chapter 4 covers valve cores and caps (Page 80) https://tinyurl.com/ybvjz3vz It will be seen that, although the polyethylene caps have no seals, the polyamide caps do. Link to comment Share on other sites More sharing options...
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