The feed valve lives under the driver's desk, one per vehicle, and limit the train pipe vacuum to 21" when the exhausters create up to 28". A picture says a thousand words - so look at the picture to see how it works. Basically the 21" vacuum supply to the driver's brake valve acts on one side of a diaphragm, and a spring on the other; when 21" is attainted, the diaphragm flexes, closing the valve between exhauster and brake valve. (I am told that bubbles only have one to supply both brake valves).
The rubber seal and diaphragms deteriorate with time and exposure to oil. Quite how oil gets in is unclear, as it shouldn't in theory! This can lead to one of two problems: if the main seal or upper diaphragm is defective, the train pipe vacuum slowly rises above 21" (over a period of a few minutes), and creating the full 28" in the release pipe will be slower than normal. If the main diaphragm fails, a leak will be heard from under the desk, creating 21" in the train pipe will be slow, and leaving the brake handle in LAP will show up the leak.
The upper diaphragm is renewable in situ, but failure of one component suggests that inspection of all three would be wise. Looking at the one on my bench now suggests that the whole innards of the valve could be removed without taking the main body out, which could be a bonus, as refitting can be very awkward.
These three rubber seals are required for the overhaul. They can be obtained via the Dean Forest DMU group.
Undo the bolts at each end and remove the valve. This can be easier said than done, depending on the layout of the equipment under the desk. In some cases removing part of the DSD assembly makes access easier. Undo the four nuts at the top, and remove the top cover to reveal the small diaphragm. Hold the end of the shaft in a vice and undo the brass nut, releasing the brass washer, diaphragm, and second brass washer. Undo the six nuts and bolts holding the two main body castings together, and separate them to reveal the main diaphragm and valve. At this point it should be obvious which components need replacement - the main diaphragm and valve suffer from deterioration due to oil, and the small diaphragm can be damaged too.
Now undo the nut at the bottom to release the spring - put a heavy cloth over it while you remove the nut, or the nut and spring location plate will get fired off at high speed! Grab the end of the shaft in a vice again and undo the large brass nut (big adjustable spanner required) to separate the main shaft into its two parts. Finally undo the strange bolt type thing with a 1/2" ratchet handle or T bar to release the main diaphragm. The main rubber seal can be prised out carefully with screwdrivers, but be careful not to damage it if it is reuseable.
It is very important to clean the whole thing out thoroughly of any oil or grease; these will attack the rubber components over a period of time. Any dirt or loose corrosion will end up somewhere in the brake system which is bad news.
Start by fitting the rubber seal into its holder on the upper half of the main shaft. Make sure that the holder is clean, particularly from oil and grease, as these degrade the rubber seal and diaphragms. Now gently press in the seal, outer edge first, being careful not to damage it. Note the lip which its outer edge has to sit underneath.
View of the rubber seal fitted to its location on the top half of the main shaft.
Now reassemble the large diaphragm. The six holes around the outside need to line up with the corresponding holes in the body castings, so put the lower half of the main shaft on the main casting before fitting the diaphragm. Now clamp its centre between the big washer (rounded edge towards the diaphragm) and the bottom half of the main shaft with the strange brass bolt type thing. You need something 1/2" square to do up this bolt; a 1/2" drive ratchet handle, 'T' bar or similar will do. Make sure you put the nut which secures the upper half of the main shaft under the bolt (thread facing upwards) before doing it up, and check the nut is free to rotate when you've done the bolt up.
Main diaphragm fitted with washer, nut, and strange bolt type thing. Make sure that the outer 6 bolt holes line up between diaphragm and valve casting! (otherwise you can't bolt it all together).
Next, refit the spring and its locating plate. This is a lot easier with an assistant (preferably 21, 6' tall, blonde but that's another story) who can compress the spring while you put the locknut on a couple of turns. (Alternatively you could make up a tool which fits into the spare .25" tapped holes in the casting) How far you do up this nut sets the train pipe vacuum, so once fitted to a vehicle it will probably need adjustment to get 21". As a rough guide, start with five threads projecting through the nut.
Spring and retaining nut fitted, with approximate adjustment made.
Now put the two halves of the main shaft together; grip the flat section at the top in a vice while you do up the nut with a big adjustable spanner.
Two halves of the main shaft bolted together.
The top casting now goes back on top of the lower casting with the 6 3/8" Whitworth nuts, bolts and spring washers. It doesn't matter which way round it goes (as far as I can tell).
Two halves of the casting bolted together, awaiting fitting of top diaphragm....
The small diaphragm can now be refitted. Place the sleeve on the main shaft (wide end first), then one of the small brass washers, diaphragm, other brass washer, and small brass nut. Grip the end of the shaft in a vice and tighten the nut, but be careful not to twist the diaphragm.
Finally, refit the top cover with the 3/8" Whitworth nuts and spring washers onto the studs, and the (seemingly optional) bottom cover over the spring, unless you're just about to fit it to a vehicle.
...and after fitting of the top diaphragm.
The most awkward part of the exercise can be refitting the valve to the vehicle, the two rubber gaskets being particularly awkward. In some cases the adjacent pipework can be loosened off a bit to make refitting easier.
I put one bolt in each end, through the gasket and pipes, and then rotated the whole thing into place. Sounds simple but in practice it took about an hour to refit the valve!
Now it can be set up - with the reservoir at 28" and the handbrake on (and the DSDs isolated so you don't have to listen to the buzzer and hold the throttle controller all the time!), move the brake handle to OFF and see what reading the train pipe gauge stabilises at. If more than 21", slacken off the nut at the bottom of the feed valve, reduce the train pipe vacuum by a few inches and recreate. I think it's something like 1 turn for 1" of vacuum. If less than 21" is registered in the train pipe (and the reservoir is above 25") then tighten the nut up until 21" is recorded, reduce the train pipe by a few inches and recreate.
Finally, refit the bottom cover (if there was one to start with), and check the setting of the feed valve in the other end of the unit. Check the two cab gauges against each other to make sure that they agree.
The completed article.