Gonzo PDWA valve

[Guest]

During the initial rebuild of my TR6, one of the many parts that needed to be replaced on my car (alright, one of every part that needed to be replaced) was the PDWA valve. The original was trashed and Apple sent it back saying to can it. New OE ones were not available, at least at the time, on this side of the pond. Moss sold me a Gonzo big PDWA valve that they claimed was for a Jag. Looks like the valve off a Spit but it is iron, not aluminum. My camera is still awol so a pic is not available but the valve is huge, necessitated rebending brake lines (not an issue, went with new Ni/Cu lines anyway) and blocked the hole in the body that was supposed to channel the tachometer cable (just went over the valve). Anyone had this issue? Does it work? I dunno, no light comes on and the brakes work so I seem to have balanced braking.

 

[BritBox]

Hi Bill,

The PDWA does not dynamically affect brake balance. It is simply placed inline to notify the driver of a failure in either half of a dual-line braking system--stands for Pressure Differential Warning Actuator, or something like that. If pressure drops on one side relative to the other, the PDWA's plunger shifts and activates the electrical switch that fires your warning light. The brake system will work fine without a PDWA, but you will not know if your hydraulic pressure turns to poo--until you hit the pedal.

I was able to rebuild mine, but I'm sure you could find a good used one on eBay or through a reputable used part seller if you want to get back to an original unit. If yours appears to work, then it probably works.

Regards,

 

[bobh]

Bill,
As mike said it is only a trigger for an idiot light. You can always remove the light from its socket and not worry about it.
You may have noticed that when you install new front pads if you pump the pedal too quickly the light will come on. that's due to the rush of fluid to the front brakes as the system tries to push the caliper pistons out from the fully retracted position. Bleeding the brakes can have the same effect. An open circuit allows the fluid to rush in the direction of the "leak".
Think of the inside of the PDWA as having a pendulum hanging in the fluid. Typically there is no real flow of fluid when you apply the brakes. If you have a leak, or an apparent leak as in seating new pads or bleeding, there is flow. That flow pushes the pendulum causing the light to come on. I have at times been able to push the pendulum the other direction by opening a bleeder and slightly pushing the pedal to create flow in the other direction (circuit). I have also removed the bulb when all else failed and I knew the system was fully functional.

 

[Scott_Hower]

I'll disagree slightly... the shuttle moves inside the housing (indicating the warning light) AND effectively splits the good brake circuit from the bad so you don't loose all of your fluid. At least thats my understanding. Thats also why the MC tank is partitioned.

 

[bobh]

Scott,
Good point. My comments are based on my experience. I never lost one of the braking circuits. Only master cylinders.
With the split master cylinder, aren't the circuits already isolated? If one circuit failed the partition in the master cylinder would prevent the failed circuit from draining all fluid? In this case I'm considering a failure to be a leak.

 

[DNK]

What is wrong with the original. Is it leaking??

 

[71tr]

The little 0-rings in the original valve which seperate the front and back systems eventually deteriorate and leake fluid which finds its way up thru the switch and onto your engine bay. Rebuilding is a simple task fo pulling it all apart and replacing the o-rings. Used units are always available on ebay price range from $30-$120 depending upon demand.

 

[Scott_Hower]

Scott,

With the split master cylinder, aren't the circuits already isolated? If one circuit failed the partition in the master cylinder would prevent the failed circuit from draining all fluid? In this case I'm considering a failure to be a leak.

They're isolated at the supply... but if the PDWA didn't seal/isolate the bad side from the good, it would let fluid from the good side out - regardless of the MC partition.

The MC partitions it, the PDWA keeps it that way. Strictly speaking, I'd also argue that it is a proportioning valve, with three possibilities: 50F/50R, 100F/0R or 0F/100R /bcforum/images/%%GRAEMLIN_URL%%/devilgrin.gif

I would never disconnect the switch or remove the bulb. If that light comes on you have a brake problem and it needs to be addressed ASAP. Something somewhere is leaking fluid.

 

[DrEntropy]

Hmmmm... I have ONE chamber in all the Brit cars' MC's. No silly LIGHTS to tell me there's a problem. Mash th' binder pedal, and iff'n it stops, I'm good. If it feels like I just stepped in a bucket of dog-squeeze, I'm NOT so good... and a fair bit o' pedal stabbin' usually ensues.

Don' need all them fancy baubles! Jus' more to have to FIX! /bcforum/images/%%GRAEMLIN_URL%%/devilgrin.gif

 

[bobh]

Scott,
My suggestion to remove the bulb is in the context of a known good system with a continous warning light on.

Without having a PDWA to section I can't argue about its ability to seal off a leaking circuit. Modern systems use a combination valve that serves as a warning switch, the proportioning valve and, in the case of disc/drum, allow the rear shoes to contact the drums before engaging the front disc brakes.
I have not seen anything to indicate the PWDA in a TR6 is a combination valve.
I'll try and find some information on the internals of the PWDA. If you will do the same perhaps we can find the answer.

 

[Scott_Hower]

Without having a PDWA to section I can't argue about its ability to seal off a leaking circuit.

I have not seen anything to indicate the PWDA in a TR6 is a combination valve.
I'll try and find some information on the internals of the PWDA. If you will do the same perhaps we can find the answer.

I have a bad PDWA for an MGB that I'd gladly have sectioned. I don't have a saw to do it though. /bcforum/images/%%GRAEMLIN_URL%%/jester.gif

There's an input on either side of the PDWA shuttle, one from each MC chamber. It has three outputs: two on one side (individual fronts) of the shuttle, one on another(combined rear) side of the shuttle. The PDWA seals one side from the other and lets the switch pin ride in a groove.

When the shuttle moves, it keeps the low side seperated/sealed from the high. It would have to, or the pressure would just re-equalize when the seal broke.

If it didn't, then I dont see the point of a dual circuit master cylinder that splits front from rear brakes. It defeats the whole purpose if the PDWA does not split the circuit and keep it split.

 

[TR6oldtimer]

The Vintage Triumph Register has this article and pictures and diagrams...

Seems like an easy fix if the internals are not pitted.

The PDWA

If your brake warning light is on it could be:
<ul>[*]Broken Switch[*]Plunger out of center position[*]Leak or other failure in brake hydraulics

[/list]

 

[swift6]

If/when the shuttle moves, it does not seal the side with less pressure. It will not block off fluid completely from passing through it. It does not act as an on/off switch. Both o-rings would have to fail at the same time to allow the front and rear to mix. The only time that either the front or the rear system will stop seeing brake fluid is when that section of the divided resevoir on top of the M/C runs out of fluid.

 

[Andrew Mace]

Right, Shawn. At the Spitfire workshop manual describes it: "In case of failure of either chamber or circuit [of the master cylinder], mechanical contact takes place within the cylinders and the remaining chamber builds up the normal pressure to operate the brakes that it controls." To further paraphrase, when the PDWA "sees" a pressure drop on one side "causes a shuttle to move from its mid-position....In the event of a partial brake failure the brake warning [lamp] system is earthed directly, causing the warning light to glow brightly." That's it.

 

[bobh]

I read the Haynes and Bentley manuals last night. Haynes includes a cross section drawing similar to the one in the VTR article as well as a photograph of the PDWA and its internals disassembled. The internals are slightly different than those shown in the VTR article.

Nothing is mentioned about sealing off the bad circuit. As the name implies this is a pressure differential warning actuator, nothing more. It is not said to be a saftey device. Only a warning actuator.

There are O'rings, or seals to keep fluid from the switch. The O'rings will help keep fluid from traversing the PDWA body to the bad circuit. Provided they are intact and flexible. However sealing off the bad circuit does not appear to be part of the design criteria, at least not beyond that which is necessary to keep the switch dry.

In the VTR photograph you can see there are no grooves to hold the O'rings. The O'rings are free to slide on the shaft and in the bore. This provides some sealing, but is not substantial enough to seal the system. For comparison look at the seals in the master cylinder.

Look at the bright side. At least we're not running the old "fruit jar" master cylinder like Doctor Entropy. Seeing the warning light gives us a split second longer to say those famous words spoken by so many just before impact.

 

[MadRiver]

One of the problems with the PDWA is that when it initially appeared on the 250, it used a different vavle/shuttle design that took strange conical seals rather than o-rings, and these are all but impossible to find. The shuttle design was changed for the TR6, though the brass housing on the PDWA remained unchanged. My PDWA started to leak a year and a half ago, and I couldn't find seals, so I tracked down a PDWA from a TR6 (hard to do given that they're all identical from the outside), and switched the shuttle and added new o-rings. No problem since then. /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

 

[ALLAN]

So I wonder is it really needed? If its causing problems why not remove the piston. When is the last time the system has failed on someone and the valve came to the rescue? I also wonder if this valve maybe the cause of the TR6 being so hard to bleed? What did the TR4A have for a saftey?

 

[MadRiver]

I can't disagree with that -- after all, my Healey and Tiger have single-circuit braking systems and no idiot lights; same for my father's XK120 and his MGTC. If memory serves, dual-circuit systems weren't mandated in the U.S. until 1968. That's a whole lot of old cars without 'em.

I fixed it only because it somehow appeals to my zeal for originality, and that I kind of think it's a nifty item.

 

[Andrew Mace]

So I wonder is it really needed? If its causing problems why not remove the piston.

It's only "needed" because of the Federal regulation that required a brake circuit failure warning light.<div class="ubbcode-block"><div class="ubbcode-header">Quote:]When is the last time the system has failed on someone and the valve came to the rescue?[/QUOTE]Never, since the PDWA has nothing to do with any failure other than giving a visual warning via the dash light.<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I also wonder if this valve maybe the cause of the TR6 being so hard to bleed?[/QUOTE]I don't think so, no.<div class="ubbcode-block"><div class="ubbcode-header">Quote:] What did the TR4A have for a saftey? [/QUOTE]Essentially, nothing, since the TR4A still had a single-circuit braking system.

 

[70herald]

Split circuit brakes were implemented because even in the event of a "catastrophic" failure, you would still have about 40-50% of braking power.
The PDWA is supposed to warn you when you have a less than catastrophic failure, such as a slow leak which will drop pressure on one side of the system.

Take a look at the old and new valves, presumably, the piston is the same diameter on both sides of the circuit. As long as this is the case, it is not going to have ANY effect on the pressure on either side. so you can safely put it in, and still have the advance warning if something starts to go wrong.
Yisrael