PDWA valve rebuild on series III - pictures

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<span>The PDWA could be buried beneath muck and dirt. A wire brushing before disconnecting pipes is a good idea, but mind the switch casing.</span>

<span>Disconnect the switch connector (black and white wires). Undo the brake pipes and let the old fluid drain off. Remove the valve.</span>

<span>Remove the end plug(s) and switch. Your first priority is to determine whether the internals can be rebuilt. If not, the valve might be scrap.</span>

<span>Use penetrating fluid, then, with a soft drift, push the valve towards the open end and out. Tapping the valve body on a wooden block also helps.</span>

<span>These are the internal assembly components. The shuttle valve and sleeves must be in good condition and the sleeves able to slide freely.</span>

<span>Remove as much muck and dirt as possible using an old toothbrush, and inspect the inner bore for excessive corrosion. See right.</span>

<span>Having cleaned the valve body, and confirmed it serviceable, treat it to a coat of paint. I use silver to make it easier to see when fitted.</span>

<span>Internals differ according to PDWA. You should renew the O-seals. Use a vernier caliper to measure internal and external diameters for new O-seal sizes.</span>

<span>I rebuilt my brakes using Automec’s silicone brake fluid, but wear gloves if using conventional brake fluid. Dunk the internals in brake fluid.</span>

<span>Liberally lubricate the valve bore. Non-silicone fluids will attack your fresh paint, so apply them with a craft brush if necessary.</span>

<span>If the shuttle valve is free of scoring and pitting in the vital areas, including the O-seal grooves, refit the two circlips to the shuttle valve</span>

<span>Next, fit the two valve sleeves over the valve, confirming they slide smoothly, then fit the two new O-seals gently into their grooves.</span>

<span>Slide the lubricated valve assembly into the valve body. Expect some resistance from the new O-rings, but don’t ever force them.</span>

<span>Locate the valve so that the central groove is visible in the middle of the switch hole. This is important to ensure the valve is centralised.</span>

<span>Clean the end plug and valve body sealing faces, and carefully fit a new O-ring to the end plug. If included, fit the yellow spacer first.</span>

<span>Affix the end plug finger tight. Fit the switch using a new O-seal and tighten carefully without damaging the O-ring. Then tighten the end plug.</span>

<span>Refit the PDWA to the chassis and plug the pipe holes until pipes are fitted to keep dirt out. Bleed the brakes using the correct procedure.</span>