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Triumph TR6 - TR6 PDWA and master cylinder issues

Can you tell me what the PDWA switch does and whether it can be omitted and replaced with inline connectors? And what happens if it fails? Does it block the fluid between M/C and callipers? I've just rebuilt the M/C and can't get the rear brake part to squirt any fluid out..Any idea how to bleed this part of the M/C?
james

James

If you go to this website it will give you all the information needed to complete your brake job. I used it and it really helped me out.


http://web.archive.org/web/20020924103959/http://buckeyetriumphs.org/

Brian
1973 TR6
Brian

James,
The Buckeye Tech site is a great place to look for information on your brake problem, but a few of the pics and diagrams are missing.
More to the point of your question. The PDWA's function is merely to alert the driver of a failure in the brake lines (a difference in pressure between the front and back brake circuits).
It can and does leak fluid if the seals are bad, but it does not block off the brake line in any way.
It can be eliminated, BUT should not be as this would present a severe safety hazard (in my opinion).
There is a shuttle in the body of the PDWA, that is connected to both the front and back circuits. When a leak occurs, and the brake pedal is depressed, the shuttle shifts toward the side with less pressure (due to the leak),and accuates the warning light switch (illuminating the dash light).
The shuttle will remain offcenter, and the warning light remain on, until you recenter it.
As to the lack of fluid going to the rear cylinders, did you bench bleed the M/C. If not it is possible for air to be preventing the fluid from being pumped through.

Mark
Mark Riddle

Does anyone know of a company who sells a PDWA and an adjustable brake bias in one?
Don K.
Don Kelly

Mark--I believe the PDWA shuttle also isolates the failed brake circuit in addition to tripping the switch. This is its basic safety function. With that in mind, one should bypass the PDWA only as a desperate last resort.

Rick O.
72 TR6
Rick Orthen

Rick,
Sorry to disagree on the isolation. The shuttle has no seals located in a position to isolate the individual hydraulic circuits from the MC/brake circuit. It has a seal to (one on each side ) to keep fluid from flowing to the electrical switch, which is not sealed.

You are absolutely correct that the "removal" of the PWDA should only be persued as a "desperate" last resort.

Mark Riddle
Mark Riddle

Hi Mark. Yes, I could be wrong and appreciate your correction, but I have read that the PDWA also maintains line pressure in one half of the system in case of failure in the other (maybe that's the case only with diagonal split systems?). Maybe that's the case only with diagonal split systems.

Next time I bleed the system I'll off-center the shuttle and see if I can push fluid through all 4 bleeders.

Rick Orthen

Rick,
The PDWA does not have any effect on pressure in either the failed circuit or the remaining (good) circuit.
Once the shuttle has shifted off-center, it merely operates the warning light, and there she'll be until you recenter.

Sorry to go on about this, but the PDWA is often a misunderstood item in the brake circuit.
Some folks believe the PDWA performs a proportioning function, this is simply not true.

Cheers,

Mark
Mark Riddle

whats the simplest method to reset the PDWA?

Bob Craske

Bob,
By unscrewing the "electrical switch", and using a small screwdriver or awl, you can easily slide the shuttle to center the small diameter in the switch "port".
Reinsert the switch, and your done.

Mark
Mark Riddle

is the pdwa switch only on American Tr6's or UK ones as well? Was it an optional extra?
james

James
I think it was on all TR6s no matter where they where destined to live. It was not optional.


To add to Marks' comments. It is likely the PDWA will need recentering if you do a major brake bleed.
Rick C
Rick Crawford

Rick C - PDWA's weren't fitted to the UK models - the warning light (with a whole lot of other stuff) never made it onto the dash panel. I'm not sure about the euro models. UK dash fascias only have the six gauges, light, wiper and rheostat switches and the two eyeball air vents! (no hazard, no brake fail etc).

We've come a long way with safety in 30 years, but I get the feeling that the safer the cars have become, the worse the drivers are. I see that 'Fly by wire' is becoming more common and quite frankly it scares the hell out of me.

R
Roger H

Roger
Thanks...you learn something every day.

I hear ya. Ever been in a cockpit of an Airbus with its' little joy stick?
Rick C
Rick Crawford

Rick- Please do not mention the "A" word around me.
Don K.
DON KELLY

Don,
Sorry. I would presume that "B" has, in some birds, a similar joy stick.

Oh Ya....dYno.....GEEESSH..make one little typing mistake and you get jumped all over!
Rick C
Rick Crawford

This thread was discussed between 17/01/2005 and 10/02/2005

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