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PDWA valve rebuild on series III - pictures

by: Auto Express team

27 Sep 2017

Go to: PDWA valve rebuild on a series III

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The PDWA could be buried beneath muck and dirt. A wire brushing before disconnecting pipes is a good idea, but mind the switch casing.
Disconnect the switch connector (black and white wires). Undo the brake pipes and let the old fluid drain off. Remove the valve.
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.
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.
These are the internal assembly components. The shuttle valve and sleeves must be in good condition and the sleeves able to slide freely.
Remove as much muck and dirt as possible using an old toothbrush, and inspect the inner bore for excessive corrosion. See right.
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.
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.
I rebuilt my brakes using Automec’s silicone brake fluid, but wear gloves if using conventional brake fluid. Dunk the internals in brake fluid.
Liberally lubricate the valve bore. Non-silicone fluids will attack your fresh paint, so apply them with a craft brush if necessary.
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
Next, fit the two valve sleeves over the valve, confirming they slide smoothly, then fit the two new O-seals gently into their grooves.
Slide the lubricated valve assembly into the valve body. Expect some resistance from the new O-rings, but don’t ever force them.
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.
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.
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.
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.