Stew Posted March 30, 2013 Share Posted March 30, 2013 I have an Autotrail Chieftain on a twin back axle al-ko chassis 1999. Just coming back from Berwick yesterday and the brakes on the rear axles started to stick on and get hot, causing me to have find a safe place to stop v.quickly. This has happened to me before and turned out to be the brake pressure retaining valve half way down the chassis on the near side just about level with the habitation door. Sure enough when I released the outlet brake pipe fluid gushed out and the brakes released. Luckily I had a spare new valve inside, which were in the van when I bought it years ago , and thought "what are these for?" I changed the old valve and replaced it with the new one and managed to get home. I have stripped the old valve down and can't see anything wrong with it, so wondered if anyone knew what could cause the problem, is it essential to have one of these valves fitted at all, and where can replacement valves be obtained without having to go to al-ko. Needless to say I will be replacing all the brake fluid with fresh, and bleeding all through. Any advice/help appreciated :) Link to comment Share on other sites More sharing options...
Ninian Posted March 30, 2013 Share Posted March 30, 2013 Hi , I had a similar problem years ago and unfortunately you have to buy the inline valve from ALKO. I can't remember how much it was but I know it wasn't overly expensive. I also have an Autotrail. Link to comment Share on other sites More sharing options...
Brambles Posted March 31, 2013 Share Posted March 31, 2013 Maybe the control rod for it was/is adjusted wrong. Link to comment Share on other sites More sharing options...
thebishbus Posted March 31, 2013 Share Posted March 31, 2013 I believe the valve is to reduce pressure to the back brakes to stop them locking and the axle winding up, yours seems to be doing the opposite. As Brambles says its operating point could need readjusting. Brian B. Link to comment Share on other sites More sharing options...
Stew Posted March 31, 2013 Author Share Posted March 31, 2013 Thanks for replies so far, the valve is an in line valve with no rods or any other means of adjustments. Inside there are a couple of springs, one ball bearing and a rubber seal and a plastic what I would describe as a valve/ jet. That's it, so can't really understand what is going wrong :( Link to comment Share on other sites More sharing options...
Keithl Posted March 31, 2013 Share Posted March 31, 2013 Stew, First to put the correct name on the item, it is a Brake Pressure REDUCING (or Limiting) Valve and not a Retaining Valve. Second to try and explain how it works. When you brake the deceleration causes the ball bearing in the valve to move forward in the valve (think of a tennis ball on the MH floor) and pushes the nylon valve into the 'jet' and restricts the amount of brake fluid and hence pressure applied to the rear brakes. Another method to control a PRV is to link it to the rear axle so when you brake and the rear of the chassis rises the valve is moved by a control rod. There is plenty of info on the internet if you want to know more, try a Google search. Unfortunately I can't help with why yours is failing other than to recommend replacing it rather than attempting a repair as if you change it's setting in attempting a repair you may find that under heavy braking you lock the rear wheels and then potentially risk losing control of the rear of your MH. Keith. Edit. To add are you sure it is the PRV at fault and not the master cylinder? If for example the pedal is not returning fully to it's 'home' position then this will prevent the return flow of fluid from the brakes when released. Try 'lifting' the pedal with your foot next time it happens. Link to comment Share on other sites More sharing options...
Brambles Posted March 31, 2013 Share Posted March 31, 2013 Ok, now we know it is the inertia type valve, is it the right way round? p.s. I call it a load proportioning valve, different names for basically the same thing. It is not to stop the axle winding up, it is to stop the rear wheels locking and skidding when braking very hard increasing the risk of the rear end 'winding' up at the front as you skid. Link to comment Share on other sites More sharing options...
Brambles Posted March 31, 2013 Share Posted March 31, 2013 Should have also mentioned the angle is important and the angle affects the rate at which it starts to operate. I have seen people tidy them up to make them level and neat thinking they are doing right. Wrong! If they are the wrong way round, then you can end up under certain comditions with then acting like a valve keeping the brakes on. What happens in the ball type inertia brake valve is the ball throws forward against a seal on a piston. The piston acts against spring, and also the piston has two diameters. The larger diameter in on the output side to brakes so in effect pressure is reduced to the brakes. You still get braking but a reduced amount. Edit ... p.s. just found this website which on 1st glance looks quite good at expaining. I have not read all yet. http://what-when-how.com/automobile/hydraulic-braking-system-automobile/ Link to comment Share on other sites More sharing options...
Stew Posted March 31, 2013 Author Share Posted March 31, 2013 Thanks again for all replies, it does give me something to look at. There is no adjustment that can be made at all to the valve. Also it is fitted level in the chassis, not inclined at all. This is how it was from the factory. As for it being the master cylinder, when I slackened the input line into the valve, there would have been pressure in the line, which there wasn't, however there was in the output line, hence the valve being the cause. Thanks again Link to comment Share on other sites More sharing options...
Brambles Posted March 31, 2013 Share Posted March 31, 2013 The mentioning of the master cylinder may not be so wild. Being level may be fine, bit it really depends on how it was designed. The problem is if level and the ball is free floating then in can easily roll against the seal on the piston. That is not what is desired, so it should be at an angle. It all depends on its design. Is there a manufacturer (or abbreviation) on the unit along with a part number or code. It really is hard to see how it stuck unless fitted the wrong way round. What may have happened though is your wheel cylinders or one of them is beginninhg to sieze. It maybe be not quite enough drop in pressure from the master cylinder allowing it to shift and just bleeding a bit of fluid allowed it to unstick. However , bleeding either side of the valve should have worked and strange normal driving would not have knocked the cylinder off if it was just the retained system pressure being enough to hold it on along with the friction in the seizing wheel cylinder. Back to the drawing board. Check the valve was the right way round, find out of it should be level or inclined. Clearly something is wrong so assume nothing. Link to comment Share on other sites More sharing options...
Stew Posted March 31, 2013 Author Share Posted March 31, 2013 I appreciate all the help and advice. I have owned this motorhome for over 8 years now, and when I got it there was these 2 new unknown items in the vehicle, these turned out to be the brake valves. I had no problem at all with the vehicle, until one outing where all the brakes on both rear axles began to stick on. If left for a while, the brakes would gradually come off on their own accord, hence believing that there was a sticking valve somewhere. On tracing the full brake system out, I found the valve which I thought was causing the problem, and it matched the 2 items that I didn't know what they were for. They came with an Alko part number, and an arrow engraved on the side showing direction of flow. Once replaced, no further problems until this last weekend, hence just replaced the valve as same initial problem exhibited. All original pipe work as it came from factory, valve fitted in correct direction, :) Link to comment Share on other sites More sharing options...
Brambles Posted March 31, 2013 Share Posted March 31, 2013 The arrow could be this way up, this way direction of travel, or indeed directiom of fluid. It could also be for the machine operator during manufactire to show which way casting is to be loaded in the milling machine. It it possible to take a picture of one and post it. You have had a valve apart., Does it look like it has a piston with two ends and ball bearing acts on one end of the piston. If this is the case then the piston should have a large diameter and a smaller diameter. The input from the pedal acts on the smaller diameter when the ball seals the centre of the piston. The larger diameter then operates the brakes. If you have twin axles then you have double the braking power of a single axle so quite a large reduction is needed so I can appreciate it will be a non standard valve made especially for Alko as they are the main provider of twin axles for conversions. A picture would speak a thousand words. Link to comment Share on other sites More sharing options...
Stew Posted April 1, 2013 Author Share Posted April 1, 2013 I will see if I can add a picture of the valve stripped down. Link to comment Share on other sites More sharing options...
Brambles Posted April 2, 2013 Share Posted April 2, 2013 Look forward to see a pic. Link to comment Share on other sites More sharing options...
Brian Kirby Posted April 2, 2013 Share Posted April 2, 2013 Stew - 2013-03-31 5:31 PM................. On tracing the full brake system out, I found the valve which I thought was causing the problem, and it matched the 2 items that I didn't know what they were for. They came with an Alko part number, and an arrow engraved on the side showing direction of flow. Once replaced, no further problems until this last weekend, hence just replaced the valve as same initial problem exhibited. All original pipe work as it came from factory, valve fitted in correct direction, :) In view of the AlKo part number, have you tried contacting the AlKo UK technical department (they used to have one, so I'm assuming they still have), and asking them for an explanation of the part and its function? Link to comment Share on other sites More sharing options...
Stew Posted April 2, 2013 Author Share Posted April 2, 2013 Hopefully a picture will be attached :) Link to comment Share on other sites More sharing options...
Stew Posted April 2, 2013 Author Share Posted April 2, 2013 Try again Link to comment Share on other sites More sharing options...
Brambles Posted April 2, 2013 Share Posted April 2, 2013 Oh! not what I expected at all. It looks like a brake fluid check valveor as you say in title pressure retaining valve. You must have a separate brake regulating valve. Anyway, I can see the arrow and I can also see how the brakes get stuck on if the ball gets stuck in the rubber seal's center hole. Link to comment Share on other sites More sharing options...
Brambles Posted April 2, 2013 Share Posted April 2, 2013 Have you got disk brakes on the rear? They are normally used in conjunction with disk brakes I believe. Link to comment Share on other sites More sharing options...
Brambles Posted April 2, 2013 Share Posted April 2, 2013 " I can see the arrow and I can also see how the brakes get stuck on if the ball gets stuck in the rubber seal's center hole." Scrub this comment of mine. Nothing to do with teh ball getting stuck. Just typed up a detailed reply explaining all about this valve and its failed to post as did it as an edit to my last post and ran out of time. Have to wait until I have time to type it all up again. Link to comment Share on other sites More sharing options...
Airstream Posted April 2, 2013 Share Posted April 2, 2013 Hi, May be this is the same item ? includes an explanation http://www.ebay.co.uk/itm/Brake-Residual-Pressure-Valve-2-PSI-Black-Brisca-F2-Superstox-Kit-Car-Rally-2PSI-/380473035882?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item5895f5cc6a Regards Ray Link to comment Share on other sites More sharing options...
Brambles Posted April 2, 2013 Share Posted April 2, 2013 You have got it in one Ray. Saves me explaining its purpose. Basically in the valve in the picture, the small ball and light spring act as a one way valve feeding the brakes and allows free flow of the fluid. When the brake pedal is released the high pressure fluid in the caliper pushes back aganst the rubber seal and ball, which is acting on the large spring. The rubber seal rests against the flange of the threaded top part sealing it off. The pressure the seal will lift of the seat is set by the larger spring and the rubber lifts of the seat and fluid flows around the sides of it and the assembly. For it not to lift something is stuck. Now it could be the rubber seal is swollen but then you would expect problems every time the brakes are applied. Another possibility is the seal assmembly is catching on the lip between the female threads and bore of the bottom half. However what is maybe more possible is there is a shoulder in the bottom half to linit the travel of the piston/seal assembly and if the brakes are released very quickly it might be the flow rate is fast enough to throw the seal assembly against the shoulder and the underside of the seal seat hits the shoulder and then forms a seal, metal to metal. However I would have thought applying the brakes again would release it and am sure applying the brakes again must have happened. So its back to the seal assembly sticking in the bore by getting caught on some shoulder or ridge. The real test would have been to release the fluid on the input side and not the output. If it did effect a cure then the valve is not faulty and fault lies further up stream with some other component. Was this actually tried? If so and made no difference then agree the valve is defective. Maybe one way to check the valve is to insert the seal and spring into the bottom half and see if there is some way you can , by pressing it down , see how it could seal fluid flow off. It seems strange there are two spare valves in the van and makes me wonder why. Has it been replaced twice already and if so, then maybe it is time to source a different manufactures part with the same pressure rating. On the other hand maybe it was a batch problem with parts and Al-Ko now supply an improved part. As brain says contacting Al-Ko is the sensible next step. Link to comment Share on other sites More sharing options...
Brian Kirby Posted April 3, 2013 Share Posted April 3, 2013 I think this valve is there because there are four sets of callipers to operate at the rear, instead of the two that the master cylinder is designed to supply. Due to the diameter of the calliper slave cylinder piston, disc calliper cylinders have quite a large fluid capacity, requiring a relatively large volume of fluid to be displaced for them to work effectively. I believe the AlKo axles retain the original Fiat hubs and brakes, and duplicate these for the second axle. I think this valve is supposed to resist the tendency for the callipers to displace back all the fluid from the rear when the brakes are released, because were that to happen the normal master cylinder stroke would be insufficient to adequately re-supply all cylinders, and the brakes wouldn't work reliably. What I'm trying to say is that it is not so much the pressure on the rear brake lines this valve is retaining, but the fluid volume necessary to operate the brakes properly. It seems to have become over-zealous, and appears now to be retaining pressure as well as volume, so causing the brakes to bind on. However, these brakes are now 14 years old, so I just wonder if the problem may be less to do with the valve itself, and more to do with the rear calliper pistons. If these are now a bit sluggish (they do "coke" up), they may not be as free as they should be, and so not generate enough return pressure to overcome the resistance of the valve. This would have the effect Stewart is describing. However, when the valve is removed, all back pressure is released, then allowing the calliper cylinders to fully retract and displace the full fluid volume. If the braking system has generally been effective hitherto, the valve clearly works as designed. When the brakes stop working it will be because something in the system has changed, not because it was initially the wrong valve. It is an AlKo design, so I really think AlKo should be consulted before any further experimentation takes place. I'm just nervous at the thought of somewhere near 5 tonnes of van with no brakes! :-S Link to comment Share on other sites More sharing options...
Stew Posted April 3, 2013 Author Share Posted April 3, 2013 Thanks for taking the time to reply to this post, just to answer a point, I did release the inlet pipe first to see if the pressure would drop and the brakes would come off, but hey didn't, when I released the outlet side they did come off. The vehicle is drum brakes on the back two axles, not discs. As I said there were 2 brand new valves in the van whe I bought it, and now over the years I have have had to replace the valve twice, I did contact Alko some time ago to see about replacement parts, but was just wondering if anybody else could supply replacements which will not carry the Alko price tag. I will look at the eBay link to see if that could be used. Now I must spend sometime digesting all the info. Once again thanks for all your comments :) Link to comment Share on other sites More sharing options...
Brian Kirby Posted April 3, 2013 Share Posted April 3, 2013 OK Stewart, for callipers read drums! :-) The principle is the same although, IMO, drums remain more problematic than discs. That valve is preventing fluid being fully displaced from the rear brakes, so that brake pedal travel does not become excessive. When you have removed and replaced these valves previously the rear shoes will have fully retracted. What I'm trying to get at is the possibility that it is that which fixed the problem, rather than any fault in the valve itself. One effect of the valve is to restrict the amount by which the shoes retract from the drums, meaning their actual movements are now very restricted. This may mean that items such as the handbrake mechanism, which acts on the shoes via a cam (or similar), as opposed to the slave cylinders, may also not be getting much actual movement. Things that don't move much are prone to stick. So, I'm wondering when was the last time the rear drums were all taken off, and the whole rear brake assembly (including the handbrake mechanism) cleaned up and lubricated? I'm not discounting the valve as a possible culprit, just that it enjoys near ideal operating conditions, being sealed from dirt and working in a lubricating bath of brake fluid. The rear brakes, on the other hand, work in an appalling environment with water and dirt thrown at them, and brake dust continually accumulating within the drums, especially if a slave cylinder is beginning to "weep". The scope for partial seizure or obstruction of movement is quite high and, as this valve further restricts the movement of the parts, it seems to me liable to increase the chances of something sticking. Such a clean up would not have been uncommon every couple of years in the days of all drum brakes. My final thought is that these drums probably have auto-adjusters. As the linings wear, the adjusters take up the slack on a kind of pawl and ratchet mechanism. This maintains what should be an optimal clearance between lining and drum. However, as the AlKo valve restricts the degree by which the linings relax, I just wonder if, once the ratchet (or a combination of ratchets) advances one further "click" the new clearance, coupled with the restricted movement of the shoe/s under the influence of the AlKo valve, might so reduce the lining to drum clearance as to allow binding. I used to do all my own drum brake adjustments, and few drums rotate perfectly circular, meaning some degree of "hit and miss" binding was almost always present when manually adjusting them - almost always requiring them to be backed off a bit for free running. So, unless you know the brakes have been stripped, cleaned, and lubricated within the past year or so, I think this may be a good place to start looking. Link to comment Share on other sites More sharing options...
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