PDWA valve rebuild

Don’t bypass a faulty brake failure switch. Gary Stretton shows how to ensure your dual-circuit brakes will work when you need them most

Making sure your Series III stops safely

he pressure differential warning actuator (PDWA) fitted to Series III vehicles gets an unfair reputation, in my opinion. Its purpose is to offer a fail-safe method of bringing a Land Rover fitted with dual-circuit brakes to a halt, should the braking system suffer rapid fluid loss and therefore operating pressure.

It does this by splitting the braking system between front and rear, providing braking capacity in an emergency failure by shutting off the faulty half of the system. Given the weight and momentum our vehicles are capable of achieving, it’s a fail-safe I welcome.

The PDWA, or brake failure switch, as it’s more commonly known, is a simple mechanism in need of sympathetic understanding. Some uninformed forum posts tell of ditching a faulty valve and replacing it with a multi-connector. This only serves to create a single line braking system so that, if the system suffers catastrophic fluid loss, only an ejector seat will save you. Not a scenario you want on your conscience. Misunderstandings concerning the PDWA perhaps stem from a lack of repair kits and not knowing the correct method for bleeding the dual-circuit brake system. The good news is that both of these are DIY friendly.

There are at least three variations on

Tthe PDWA valve fitted to Series vehicles. For example, some versions include a plunger ball for the switch, and a twopiece valve (as shown in the SIII workshop manual). The valve shown here is the single-piece valve without plunger ball, which is fitted to some SIII civilian and military models, and the 101 Forward Control. So be guided by the internals you find when the valve is dismantled. The inspection and rebuild procedure is similar for all types. I couldn’t find a service kit for my valve but, if you do, be wary of New Old Stock (NOS) kits. Aged rubber O-rings still deteriorate in storage, so source new rubber items, regardless.

There are two types of PDWA valve assembly: One-piece valve type, with top-mounted switch; and two-piece valve type with low-mounted switch and ball (valves are in yellow, O-seals black, switch green, and ball red).

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

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

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.

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.

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

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

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.

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

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

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.

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

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

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.

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.

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

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.