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daveturnbull

5 port PDWA alternative?

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Question on behalf of my dad. His '84 series has just failed the mot on the PDWA valve. The part number is NRC4270. It seems this is no longer available to buy new. He's not really keen on rebuilding stuff like I probably would. Is there an alternative part that would fit and do the same job? 

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I'd probably just delete it with a couple of T-pieces, or new lines if they aren't in good nick.

The repair kit is probably quite cheap....?

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 I've built a couple, they aren't difficult, but a faff.

Two tee pieces is a good plan, but also get a reservoir level float switch as well, and wire that in to the brake failure circuit.

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Oh, I couldn't find a kit, but the details of the parts are on my lightweight build thread.

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That's a PDWA, not a reduction valve.

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Yes, un screw the  switch, fill the hole with silicone grease. Fill the top of the switch with the same. It will add years to the life.

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Posted (edited)

Mine never had one. These came fitted to late S3 only (I think?).  Herd-wisdom is that these are more faff than they're worth, hard to bleed "Just remove it" etc. Agreed, I've never had one, but like so much LR folklore, I'm not sure this stands even mild scrutiny? Quite the opposite, I'd fit one.

Tell me where my thinking goes awry:

1) It's a safety device - put there for good reason. You hope you never need it. Safety devices are always a faff. And?

2) We can bleed systems far more complex that anything on a Series. If we can't bleed a Series, especially with a vacuum/pressure-bleeder this marks incompetence. Not much to do with the PDWA?

3) It worries me that you can't buy the rebuild kit easily. Is this mindless penny-pinch herd-wisdom or is there real foundation?

Where I lack, I'd fit one, or make what I have work - certainly never remove? Who's right here?

 

 

Edited by Landrover17H

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@Landrover17H - it's a warning device, it lights a light on the dashboard (in theory) although I'm willing to bet very few of them are actually functioning by now, either through seizing up, poor contacts, or having the wire removed to make bleeding easier.

It warns of a broken brake circuit, although as I believe the vehicles with them had dual-circuit brakes I'm pretty sure you'd notice the loss of braking performance (and brake fluid) without a little red light. It does nothing to help with brake failure or safety besides that, and ultimately introduces an extra point of failure into the brake circuits by adding joints.

My 109 had one, I never re-fitted it with the disc conversion and having had an actual brake failure since due to damage I can say I definitely noticed it without the warning light :lol:

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Posted (edited)
Quote
 

@Landrover17H - it's a warning device, it lights a light on the dashboard (in theory) although I'm willing to bet very few of them are actually functioning by now, either through seizing up, poor contacts, or having the wire removed to make bleeding easier.

It warns of a broken brake circuit, although as I believe the vehicles with them had dual-circuit brakes I'm pretty sure you'd notice the loss of braking performance (and brake fluid) without a little red light. It does nothing to help with brake failure or safety besides that, and ultimately introduces an extra point of failure into the brake circuits by adding joints.

I accepted what it does, and how it works. And its limitations. In the finish we see a red light, not much else? Thus you see a red light one application of the brakes (or maybe  a few seconds) before you'd likely 'feel' the issue? Which confirms the problem quicker, might be crucial in some conditions? What it isn't, is proper dual-circuit, fail-safe brakes. It warns you, you're on one circuit, and seconds quicker than you might otherwise notice? But a start, no?

Never had one, hence I'm unqualified. If we accept that this is all the 'thing' does, and not neglected with un-flushed acidic-fluid in there for years - as many an owner's wont - are they really that bad?

Edited by Landrover17H

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Only fitted to dial circuit systems, failure is usually corrosion in the barrel, and o ring failure.

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Be better off fitting a low fluid level warning cap to the reservoir.

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Posted (edited)
1 hour ago, Bowie69 said:

Be better off fitting a low fluid level warning cap to the reservoir.

There you have a strong point,  I would think a float-level indicator would come on a bit later than one fitted to the PDWA, more - 'as well as', than 'instead of'?

Have I got this right?  The advantage of the PDWA is that it'd fire even under light braking, when you'd not quite so quickly notice the issue, (or be paying attention) over finding out when used in anger? That could be minutes earlier? Finding your brakes gone at the top of the hill, or the bottom? Seems to me, these things get ditched because they're pricey, and awkward, and the limited degree of safety they offer gets used to justify?

Broadly because any fitted from new are in pretty poor condition now, and in this condition, any advantage they once offered vanishes, or could even make things worse? Tight-fistedness, no understanding of their timing and operation, and most in poor condition now etc, doesn't make them bad. These were fitted with good-intention. With these caveats,  all  equal, and on balance, properly maintained a PDWA can only be a good thing?

The problem is, they  weren't made to last 30-40 years hence there's a thousand blokes saying remove them. Or - one in 5000 thankful in the extreme, their LR has one.

Am I allowed to call this one 'Myth Busted'?

Edited by Landrover17H

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It always makes sense to join 2 separate circuits with a safety device - which itself can leak to the outside world.

What warning will it give you?  Its operated by a shuttle valve which operates the switch.  The shuttle valve will only move when there's a greater pressure on one side than the other, ie when there's a leak from one circuit.  However its got to be quite major to be operated - a slightly weepy cylinder won't leak enough to operate it.

 

The thing was introduced to Land Rovers initially with the optional split circuit system - which I think was compulsory on many diesels - in that they were the only ones available.  That system is a bit different to the post 1980 system - which splits the front brakes with a junction at the back of the front crossmember rather than making the split on the PDWA.  This is the difference between a 4 and a 5 port device.

 

 

 

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a slightly weepy cylinder won't leak enough to operate it.

I take your point, this device is not perfect and if I'm expecting it to report all brake-faults, in an instant  I'm set for disappointment. You're correct, a weepy cylinder won't instantly fire the switch, yet will when we need it to, one or two prods, away from critical. Not ideal, but that's not a bad thing is it?

This device does add further 'risk' with more joints etc, but these risks would be less likely, and indeed if a joint failed will probably throw the switch anyway? On balance I still can't see how, working  as intended we're worse off with a PDWA?

However, if I'm not prepared to trouble with repair of my now corroded and non-functioning PDWA, the "Mine's never had one, managed for 40 years without one." logic stands. Better removed.

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On 7/26/2020 at 7:48 PM, Bowie69 said:

Be better off fitting a low fluid level warning cap to the reservoir.

Exactly what I did on my 109.  The PDWA is still fitted,  but even though I installed a brand new genuine one during my rebuild, I don’t trust it at all.  The pressure differential needed to activate it is probably quite large anyway - you need to be braking quite hard and have a complete failure on one circuit for it to illuminate the warning lamp, IF it’s working correctly.  It strikes me as a part fitted to tick a regulatory box, not to serve any practical purpose.  A level sensor is far more effective.

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Posted (edited)
Quote

The pressure differential needed to activate it is probably quite large anyway - you need to be braking quite hard and have a complete failure on one circuit for it to illuminate the warning lamp, IF it’s working correctly.

 I would have thought the pressure-differnetial was more than "quite large", nay, I'd have it as vast? I'm truly struggling to name one place we routinely come across greater pressure-differentials? 70+ Bar (very light braking) in the working circuit is quite a bit, as against 1 Bar in the failed circuit? That's up at 100Bar (1500 psi)  under hard-braking. How much pressure-difference did we want? Enough to push the diddy-switch anyhow? I have a paint-ball/scuba-compressor here, it'll do 250Bar, (300Bar at a push - 4300psi) but not too many have one of those.

It had occurred to me that rather than the goodness of any bean-counter's heart, the PDWA fitment smacks of  'Chapter 4 - subsection iv' .Fitting it would not have been a light decision. When LR buys for a production-run, even £4-5 per part, becomes a lot of cash where you don't have to spend it.

The cynic in me sees the regulatory requirement secured a bureaucrat's pension, but you'd hope it also pushed manufacturers to a level-playing field. The big issue for me is that you can't check PDWA operation easily - Snagger, you have one, but short of cracking-off a bleed-nipple, there's  no easy way to know if it works? It is a device you have to 'hope' works? A float-switch is easier to see and check for operation.

 

Other than neglect, and tight-fistedness,  with a heap of non-comprehension for its correct operation, I've not seen the argument to withstand any scrutiny to support herd-wisdom, ie:  Our PDWA device should be removed.

I claim 'Myth Busted'.

 

Edited by Landrover17H

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If you trigger the PDWA unit, you’ll have to strip it to reset it - it’s not self returning, even if you match the pressures on each circuit.

I think it’s a fair assumption that most are seized.  They shouldn’t move in service, unlike master and slave cylinders, so aren’t self clearing.  It think it’s an interesting point that I have never triggered it when bleeding brakes, either manually or with a pressure bleeder at 40psi+, and have never heard of anyone triggering one in any circumstance.

To avoid legal issues, I decided to leave mine installed and wired up.  I think it makes bleeding much harder as it’s an air trap, but once that is done, it doesn’t have any impact on the system.  The level sensing cap was simple enough to add into the system as an extra sensor.  The system normally uses the PDWA, servo vacuum switch and the warning light test trigger, with some vehicles also having a hand brake switch (Swiss, I think, and maybe a few others and possibly a customer option) and a pedal travel switch (mounted on the pedal box, works like the brake light switch and is also uncommon).  From a legal perspective, it’s much easier to add a warning sensor than to remove one...

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Posted (edited)

Oh, as I said I've never had one, and never played with such. Not knowing what I'm talking about has never stopped me before? I have it that a PDWA is a 'shuttle', thus in proper working condition would reset as soon as pressure equalises? If it won't even reach to that, these things are junk.

I see you point, in normal use it won't be moving. If a PDWA has been sitting in tired, and thus acidic fluid, it'll be seizing very quickly. A brand-spanker could be 'done for' inside 3-4 years whereas MCs and slaves, simply because they move, will be more tolerant to poor upkeep.

An aside: I've had experience of seized calipers not moving with my foot hard down. The pressure must have been vast. I've had to force them to 'let-go' by connecting my scuba-compressor. The sound is nothing less than a 'gunshot bang' when they finally let go. I find it easy to see a PDWA seizing under far less.

I can undestand that maybe these things need more than 40psi (2-3Bar) to fire in normal -operation. We've 70Bar (ish) under light braking, to play with. If the other circuit is only starting to weep, there might only be a differntial of 10Bar (150psi), and if the PDWA is  part-seized it won't be going anywhere.  However if the failing-circuit is weeping, we've likely still got some kind of brakes, and here your float-switch would fire first. Which again rather condemns the PDWA?

Snagger, you've experience of these things, do I get this right? Are you saying a PDWA  won't reset in correct operation? If they don't, my attempt to Myth-bust is on very shaky ground. But, if fitted, I take you point on leaving them in. Removed, there's a sharp-eyed loss-adjuster waiting to have us for breakfast. Whereas, adding a sensor isn't likely to be an issue.

 

I don't know why I care, I just like to know this stuff.

Edited by Landrover17H

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It won’t reset unless you remove the end unions or plugs to manually move the shuttle. It has no internal springs and it doesn’t isolate a leaking system, so a circuit failure will cause the shuttle to move to one side (triggering the switch) and stay there.  Restored pressure on both sides will not move the shuttle as the pressure is equal on both sides of the shuttle.  If you apply more pressure to what had been the failed side to move the shuttle to the centre, unless you have a method to meter the correct volume of fluid doing so, it’ll overshoot and trigger again.  Contrary to popular rumour, they don’t shut off a leaking circuit.  I agree that they’re useless.

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With one or two large caveats, my blatant attempt to 'Myth-bust' fails. Even if the standard reasons given for removal don't hold water, with such a laundry-list of stuff against the PDWA, for once, LR folklore actually holds water. The problem  is not the myth, it's the faulty herd-logic and expectations used to support it.

By turn about, I think the myth IS properly busted. Your point about removal putting  us on thin-ice for legal reasons, ie: 'removal of a safety device'  does stand  scrutiny. I will admit I'm miffed. I've succeeded in busting LR folklore in the past. Oddly this myth still got busted, just not by me.

In short: An item of dubious one-use value it may be, yet, unless we want to run the risk of falling-foul of a loss-adjuster, we'd be wise to leave our PDWA on.

Edited by Landrover17H

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My apologies, I see it as four lines of thought...

i) LR folklore says it should be removed, but usually for the wrong reasons.

ii) My proposal was that the  PDWA has a value. This value is at best, more dubious than I'd thought.

iii) Despite its detractors, it's a great device,.

iv) No it isn't - It's junk.

But it's all made into nonsense: Points i) to iv) are an irrelevance,  the insurance reasons you gave settle the point. Insurers are highly unlikely to sanction removal of this 'safety-device' . Regardless of the truth, your points or mine don't matter, LR folklore loses. The PDWA stays anyway.

Edited by Landrover17H

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