PDWA valve rebuild on a series III

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

The 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 the PDWA valve fitted to Series vehicles. For example, some versions include a plunger ball for the switch, and a two-piece 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).

The ‘Brake Test’ switch on the dashboard is an instant system check. If it lights up with the ignition switched on, the system should be functioning correctly because the shuttle valve is centralised in the valve body. If not, then check for fluid loss and that the brakes are correctly bled. Also check for 12 volts at the switch connector contacts, check the bulb, and the earth connection.

If the Brake Test switch doesn’t illuminate, check that the valve is still fitted and has not been bypassed using a four-way connector. Two wires, both black and white, run to the connector from the loom, so follow them to the PDWA. If buying a Land Rover with a Brake Test switch, check it is working before agreeing a sale price, and haggle accordingly.

Bleeding the breaks

The shuttle valve must stay centralised so don’t press the pedal until the brakes are bled. If it moves, triggering a system fault, centralise it again by opening a bleed valve and removing the switch (step 14). To bleed, open the bleed nipple on the furthest brake cylinder from the master cylinder (on a right-hand drive car, that’s the left-hand rear). Then, in sequence, bleed the right hand rear, left hand front and finally the right hand front. It’s much easier with a Gunson Eezibleed. Finally, check the Brake Test switch to confirm the valve is functioning correctly.

Tools: Spanners, wire brush, thin screwdriver, brake bleeding kitTime: two hoursCost: £2 for seals

Health and safety 

• Brake fluid will eat into paintwork, so avoid spills. • Wear eye protection and gloves when handling brake fluid and working on brake lines. • Clean the parts, and ensure no dirt can get into the system. • Always investigate if the brakes aren’t functioning correctly, and if in any doubt, ask your garage.

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