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.

Partnership
Need to sell your car?
Find your best offer from over 5,000+ dealers. It’s that easy.

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.

Recommended

Range Rover thefts prompt JLR to boost security on old and new cars
Range Rover action
News

Range Rover thefts prompt JLR to boost security on old and new cars

Range Rover owners will have extra protection against car thieves and the tech can be retrofitted to older models
22 Nov 2023
Best new cars coming in 2024
Best new cars coming in 2024 - header image
Best cars & vans

Best new cars coming in 2024

There are some big new models from the likes of Audi, BMW, Citroen, Ford, MINI, Skoda and more on the way in 2024
20 Nov 2023
The scariest cars we've ever driven
Scariest cars we've ever driven - header image
Best cars & vans

The scariest cars we've ever driven

The Auto Express team have cast their minds back to the scariest cars they’ve had to endure
31 Oct 2023
‘JLR’s new CEO is just the man for the job’
Opinion - JLR CEO
Opinion

‘JLR’s new CEO is just the man for the job’

Editor-in-chief Steve Fowler believes that JLR’s Adrian Mardell is the right CEO at the right time
16 Aug 2023

Most Popular

‘Forget leasing a car, a cut-price van should be your next everyday vehicle’
Opinion - Fiat Scudo
Opinion

‘Forget leasing a car, a cut-price van should be your next everyday vehicle’

With some huge savings to be had, Mike Rutherford thinks a van could be the perfect vehicle
3 Dec 2023
New Renault 5: price, specs, launch and on sale dates
Renault 5 EV concept at 2022 Goodwood Festival of Speed
News

New Renault 5: price, specs, launch and on sale dates

Renault’s reborn Renault 5 will start from €25,000 in Europe, suggesting a circa-£30k price in the UK. It's due on sale in the summer of 2024. Here's …
1 Dec 2023
Deal of the Day: Vauxhall Grandland is an ideal family SUV for £210 per month
Vauxhall Grandland front cornering
News

Deal of the Day: Vauxhall Grandland is an ideal family SUV for £210 per month

Vauxhall’s largest SUV can be had for the same price as most superminis, making it our Deal of the Day for 1 December
1 Dec 2023