George Collings Posted April 3, 2013 Share Posted April 3, 2013 While drum brakes have return springs that pull the shoes away from the drum with disc brakes the pads are just knocked back clear of the disc by run out and/or slack in the wheel bearing. I doubt Al-Ko models with disc brakes have a pressure retaining valve as this would prevent the pads moving clear of the disc. Its a different matter with drums In addition to the general servicing brake mentioned by Derek consideration could be given to a new set of brake show spring as they may have weakened with age. Pressure limiting and load compensating valves are not just to restrict brakeline pressure to prevent skidding. They also ensure that all the axles do an appropriate share of moderate braking. This is important in prolonged braking situations. Typically in an alpine descent where a fully laden rear axle set up to not lock in an emergency when lightly laden would throw all the effort to the front causing overheating and brake fade. The driver brakes harder to compensate and possibly locks the rearsand/or soils underpants. Link to comment Share on other sites More sharing options...
Airstream Posted April 3, 2013 Share Posted April 3, 2013 Hi All, Clarification Disc/Drum install Install the valve as close as possible to the master cylinder and position in the direction of brake fluid flow – the valves are marked with an arrow to identify direction of flow. The inlet side is located toward the master cylinder side and the arrow marked end is located towards the brakes. A 2 PSI valve is designed for disc brakes and a 10 PSI valve for drum brakes . If the vehicle has a single outlet master cylinder and a disc/drum system, a 2 PSI valve is required in the disc brake line and a 10 PSI valve in the drum brake line (after the tee fitting). Regards Ray Link to comment Share on other sites More sharing options...
Brambles Posted April 3, 2013 Share Posted April 3, 2013 "I doubt Al-Ko models with disc brakes have a pressure retaining valve as this would prevent the pads moving clear of the disc. Its a different matter with drums " They must be used with disk brakes where the calipers can be higher than the master cylinder. This is because the fluid can drain back from the calipers and withdarw the pistons from the pads. There are also other situations where they are required where vibration can knock the calipers pistons back. Drum brakes are different because they have springs in the wheel cylinders forcing the pistons out. They are also often used inline with front disk brakes where applying the hand brake allows the wheel cylinders to expand drawing in fluid. The valves prevent fluid being drawn from the front disk brakes rather than the master cylinder. On vans and trucks with long wheel bases then there are frequent occassions when the rear brakes can be higher than the master cylinder and reservoir. Even on cars this happens when placed on a transporter the valve may not strictly be required but will if being transported at an acute angle. edit - some typos. Link to comment Share on other sites More sharing options...
Brambles Posted April 3, 2013 Share Posted April 3, 2013 I think by now it is fairly conclusive the valves are faulty and intemittently failing possibly as they age. Maybe it is related to weak springs in the rear brakes but then it would happen every time the brakes are applied. They recover after the fluid pressure has been released and this indicates teh valve is sticking closed under pressure. Now maybe it is the piston inside the valve travelling too far and sealing at the opposite end or maybe it is by some other method and without actually seeing a valve in detail I could not possibly be certain. One thing for sure it seems, the valve is misbehaving. I am sure the design must be rigourously tested, leaving the copper washer as a possible simple un inspected variable in production. Maybe it is too thin or indeed too thick a washer. Maybe there should be some grooves inside and a machine operater overlooked that stage of machining,,,who knows. My next step would be to confirm the pressure required and replace with an alternative make from a reputable manufacturer. It is actually not too hard to determine the pressure the existing ones open at with a bit of intuity. Link to comment Share on other sites More sharing options...
Stew Posted April 4, 2013 Author Share Posted April 4, 2013 All brake shoes, hubs, cables, etc fully stripped, cleaned re assembled and adjusted by my own fair hands last year. So I am happy there are no issues or leaking braking cylinders, I have the van MOT'd by VOSA yearly, and the brakes are fully tested on their machine, with no issues, last done Nov 2012. I will contact Alko and see what they have to say. Thanks for all advice :) Link to comment Share on other sites More sharing options...
Brian Kirby Posted April 4, 2013 Share Posted April 4, 2013 Good! That eliminates that possibility, so it has to be the valve. Maybe it just has to be replaced every so often due to fatigue/wear? It is interesting that there were two in the van when you got it, which seems to point that way somewhat. Hope you get it fixed without too much further expense/delay! Link to comment Share on other sites More sharing options...
George Collings Posted April 4, 2013 Share Posted April 4, 2013 Brambles, Many thanks for the update, I am getting out of touch and confirms I was right to give up editing Interchange four years ago. I will stick to rebuilding my 1947 Riley RM or go play with the lathe I bought for my 70th birthday. Link to comment Share on other sites More sharing options...
Brambles Posted April 4, 2013 Share Posted April 4, 2013 George Collings - 2013-04-04 10:17 PM Brambles, Many thanks for the update, I am getting out of touch and confirms I was right to give up editing Interchange four years ago. I will stick to rebuilding my 1947 Riley RM or go play with the lathe I bought for my 70th birthday. Please don't just stick to your projects - I enjoy your posts and am sure you give much more assistance than you give yourself credit for. We all get things wrong sometimes and its how we learn when we get confused and who says I am always correct. Talking of Lathes, I would love to have one, both a woodworking and a small metal working lathe. Am wondering about some of these small hobby ones. If you can give any advice on a good miniature lathe I would be grateful. I want to maybe make parts up 3 inches long and maybe up to 2 inch diameter and be able to cut threads on it. Link to comment Share on other sites More sharing options...
Caddies104 Posted April 5, 2013 Share Posted April 5, 2013 Hi Back in my apprentice days, which is a long, long time ago, I remember a small Lathe called Myford... Seems they are still around.... So if you Google it, you will find some for sale Derek Link to comment Share on other sites More sharing options...
George Collings Posted April 5, 2013 Share Posted April 5, 2013 Brambles Lathes are a bit off topic I will PM you. If anyone is interested I will start a new thread in Chatterbox. Link to comment Share on other sites More sharing options...
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