Helen B Posted February 3, 2022 Share Posted February 3, 2022 Hi there folks Hope you can help.... We have a 1986 Bedford MJ and the orange light and buzzer (which are related to the air over hydraulic brakes) are staying on all the time! The pressure in the gauges read 7 and we've been told it should be more like 8 or 9. Any ideas? Many thanks in advance Helen and Phil Quote Link to comment Share on other sites More sharing options...
Sean N Posted February 3, 2022 Share Posted February 3, 2022 I can't recall the set pressure (it's been a while) but I'd have thought if gauge and warning agree they're probably right, and if it's making some air but not enough, the problem could be leaks, compressor worn (less likely, but the valves might not be sealing properly) or an unloader valve problem (setting or more likely just leaking or sticking). I have found on R and M types that have been stood for a while that diaphragms in some of the valves can split causing small air leaks. First thing to do might be a visual check for piping problems, particularly on any plastic lines that might have aged, then if you have access to one, attach a compressed air line to the system and charge it so you can listen for leaks without the engine noise, or failing that charge it using the engine, shut off and listen for leaks a section at a time, recharging as required. Quote Link to comment Share on other sites More sharing options...
Helen B Posted February 3, 2022 Author Share Posted February 3, 2022 Hi Sean, thank you for reply. We have been trying to identify where there may be a leak - been spraying soapy water everywhere. Can't understand why the buzzer doesn't go off??? Could it be a faulty switch itself do you think? Would you think it's 2 separate issues- 1) buzzer not going off and 2) not reaching the right pressure Any help is much appreciated. Helen and Phil Quote Link to comment Share on other sites More sharing options...
john1950 Posted February 3, 2022 Share Posted February 3, 2022 (edited) If the air tanks get to the correct pressure the buzzer will go off. Going back a long way in my memory when the air get up to pressure the unloader valve lifts and keeps the inlet valve in the compressor head off its seat with a jet of air. These unloader valves get a carbon build up inside and unload prematurely if you follow the small pipe from the compressor head you should come to it, it is adjustable. There should be a non return valve in the main pressure pipe to the tank it will be a long connection between two pipes, this also carbons up but can be cleaned. If you take them apart they have springs inside so be carful or you will spend a lot of time looking for them. You can check if it is the unloader valve by disconnecting the small pipe and then see if you have a better air pressure, if the buzzer stops sounding then you are on the right lines. There is a max pressure valve on the tank so that will unload if the pressure gets to high. Clayton Dewandre valves are still available if you shop around. Edited February 4, 2022 by john1950 addition Quote Link to comment Share on other sites More sharing options...
Helen B Posted February 5, 2022 Author Share Posted February 5, 2022 Hi John Many thanks for your suggestions. We'll have a look and see where we get to. Cheers Helen and Phil Quote Link to comment Share on other sites More sharing options...
Sean N Posted February 5, 2022 Share Posted February 5, 2022 Helen, Phil, The way I look at it is this. You need to approach the problem methodically. There can only really be one of three basic problems: (1) the vehicle is not making enough air; (2) the vehicle is making air, but it's escaping; (3) the pressure indication and warnings are wrong. To take (3) first, the pressure switch could be faulty, but they are two separate systems and for both to be faulty the same way at the same time is unlikely (though not impossible). Assuming what you've been told about the pressure is right, your visual indication (gauge) and warning (light and buzzer) agree, which suggests the buzzer isn't the problem. I don't have my Bedford books handy and it's been some years since I worked on air over systems, but it would be good to confirm the correct pressures. It might be that the vehicle isn't making enough air, and that takes you down the route of the sort of compressor and unloader valve problems I mentioned and which John has expanded on, as well as more obscure problems like blocked intake, flexible pipes collapsing internally, etc. In my experience leaks are quite common, particularly if the vehicle doesn't get used much, and it's also quite easy to check for leaks and confirm or eliminate them; whereas governor valve or compressor faults mean taking things apart (and as John says risking losing bits). That for me makes checking properly for leaks the first thing to do because it's the easier. The advantage of charging the system from a separate compressor, if you have the option, is that it makes it easier to check for leaks but also it means that you can get the system up to pressure and confirm the pressure gauge and warning systems are correct. If you are fully confident there are no leaks anywhere, then compressor and control system are the next to check. Some additional questions. Does the pressure get up to 7 bar quickly, or does it take a long time? Are you running the engine at idle to build up the pressure? What happens if you hold the engine at fast idle or about medium revs? Has this vehicle been unused for a long time, or is it known to have been working properly recently? Quote Link to comment Share on other sites More sharing options...
Helen B Posted February 5, 2022 Author Share Posted February 5, 2022 Hi John Many thanks for your detailed post. We bought the truck last May and it was fine - both buzzer and light went off - its only of late that both are starting to stay on.. As for running, we have fired it up and moved it about the yard maybe fortnightly; however its not been on a run /used regularly . We've asked on a few different forums and FaceBook pages in relation to the pressure and quite a number of people agree that it should be between 8 and 9. Although we don't have this confirmed in a manuals and to be honest it may well have gone up to 8 when we first got it - we can't remember! Recently the pressure seemed to build much slower than it used to. One gauge lags behind significantly now and from memory we're sure it didn't do that., but both don't go beyond 7. We've tried running the engine both at idle and fast idle and it doesn't make any difference; nothing seems to make a difference. We'll have to have a proper look (and think) and we're just wondering if we can get enough pressure from the compressor in the workshop to test as you've mentioned. Many thanks again Helen and Phil Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted February 5, 2022 Share Posted February 5, 2022 Behind the front grille on the nearside is a small air reservoir that we referred to as the 'wet tank', this has an automatic drain valve at the bottom and it can leak, due to a oil and condensation, with the pressure up as high as it will go, try feeling under the valve to see if it is leaking air. Quote Link to comment Share on other sites More sharing options...
Helen B Posted February 6, 2022 Author Share Posted February 6, 2022 Hi Richard Thank you for the suggestion. Phil's had a look at this and it doesn't look to be leaking. He pressed the 'release valve' and some oil and water came out the first time. We believe there's filter in there - do you think this could be blocked up and stopping the air going to the main tanks? Cheers Helen and Phil Quote Link to comment Share on other sites More sharing options...
john1950 Posted February 6, 2022 Share Posted February 6, 2022 If you look on this site for -bedford air brake problem, bedford trucker was having the same problem. There is also a diagram of the air system in the post. Quote Link to comment Share on other sites More sharing options...
ltwtbarmy Posted February 6, 2022 Share Posted February 6, 2022 Quote Link to comment Share on other sites More sharing options...
john1950 Posted February 6, 2022 Share Posted February 6, 2022 (edited) If you read the post you will find Richard Farrant was giving valuable assistance back than as he still is today. Also people like N.O.S. that take the time to look out original diagrams and notes to aid trouble shooting. They make this forum stand out from the crowd. Edited February 6, 2022 by john1950 addition 2 Quote Link to comment Share on other sites More sharing options...
Helen B Posted February 6, 2022 Author Share Posted February 6, 2022 56 minutes ago, john1950 said: If you look on this site for -bedford air brake problem, bedford trucker was having the same problem. There is also a diagram of the air system in the post. Hi John We've literally just come across this 🙂 thank you for pointing it out to us. Helen and Phil Quote Link to comment Share on other sites More sharing options...
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