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PRESSURE DIFFERENTIAL WARNING ASSEMBLY

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jamesregis Avatar
jamesregis Gold Member Jim S
Kettering, OH, USA   USA
I think my brake PDWA piston is froze inside the valve assy. I suspect this because my brake system is new yet my braking is very weak. I understand that this H shaped valve assy protects me if either the front or rear brakes spring a leak.If I am not concerned about having this feature, Is there any other down side to just removing the piston?

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Yellowhawk Valley Avatar
walla walla, WA, USA   USA
1969 Triumph Spitfire "Walla Walla"
1969 Triumph Spitfire "Portland"
1972 Triumph Spitfire MkIV "Spokane"
1975 Triumph Spitfire 1500 "Dayton"    & more
Expect a lot of varied responses to the question.

My suggestion is, if you are considering taking it off, first open it and see if you are right. The unit has no bearing on the operation of the brakes (and yes there will be others that disagree with my assessment). All it does is turn on a light to tell you there is an issue, it does not block the flow of fluid the to the brakes. Pull the light switch and check to see if the shuttle is off center, if not put it back. If it is off center, use thin pointed tool to reach in and move it back. (You may need to pull the little ball out of the hole first if yours is the style that uses one in there.) If off center, go find out what caused that issue.

Most often bad brake response is due to worn master cylinder, worn hoses that flex/balloon when they are applied, bad shoes/pads, or clogged pistons in the front or rear units.

Good luck with it.
Dan

carChips Avatar
carChips Victor Harnish
Kelowna, BC, Canada   CAN
1933 MG Magnette
1973 Triumph Spitfire 1500 "Chip"
1989 GMC Sierra 1500 "Bush Truck"
If your brakes are weak, I'd suspect air in the system. Do you have a firm pedal?



'S all for now
Vic

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spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
Jim,
There is a major downside to removing the PDWA piston. It defeats the separation of the dual circuit brakes. Better to do as Dan has detailed, or bypass the PDWA by coupling the front input line to the front output line and the rear input line to the rear output. My preference is to run a new line from the master cylinder to the first fitting in the line beyond the PDWA in each circuit.
All the best,
Paul

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
In reply to # 1502172 by spitfire50 Jim,
There is a major downside to removing the PDWA piston. It defeats the separation of the dual circuit brakes. Better to do as Dan has detailed, or bypass the PDWA by coupling the front input line to the front output line and the rear input line to the rear output. My preference is to run a new line from the master cylinder to the first fitting in the line beyond the PDWA in each circuit.
All the best,
Paul

The real danger of removing the shuttle is that the plastic switch for the PDWA will not stand brake fluid pressure. It will leak or pop out completely leaving you with NO brakes!

By-pass the PDWD with new line as paul says or permanently plug the centre link of the PDWA with threaded, loctite-ed plugs.

I too suspect you poor braking lies elsewhere.

Is the brake pedal firm (not spongy )?

How well does the car stop if only the handbrake is used?

If you brake hard on a loose surface, which wheels lock first?

CraigL Avatar
CraigL Gold Member Craig Larsen
Santa Clara, CA, USA   USA
Dan - you are saying if the brake warning light is on, you can center the PDWA shuttle switch directly to turn the light off rather than sightly bleed the front/rear brake systems to center the switch? I have no brake issue, but nice to know there is an easier way to center the switch on my 1975 Spit. Thanks, Craig

trrdster Avatar
trrdster Wayne Tate
Spencer, NC, USA   USA
If and only if, the electric switch is leaking and the rest of the system seems OK.
A good fix is a bleed screw to stop the leak and also hold the scuttle in place. This will work on the early ones with the ball bearing.
Had to do a emergency fix for a friend a couple of years ago and he is still running with that fix. He says when his car doesn't stop, he will know something is wrong, go figure that.



Wayne
1970 TR6
2000 Jaguar XK8
1949 Triumph Roadster 2000
1978 Spitfire (rust victim)
1971 GT6 (tarp covered for 12 years, rusted inside out)
1980 Spitfire (getting all the good GT6 parts, all poly suspension and Spax shocks)

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
My experiance has been the if the shuttle is activated, consdering the age of our cars, the 'O' ring seal on the shuttle will somtimes tear and you end up with a leak at the switch.

Of course, the nature of the PDWA is, you don't know you have lost your fluid until you hit the brakes.

So much for the PDWA being a safety device!

Yellowhawk Valley Avatar
walla walla, WA, USA   USA
1969 Triumph Spitfire "Walla Walla"
1969 Triumph Spitfire "Portland"
1972 Triumph Spitfire MkIV "Spokane"
1975 Triumph Spitfire 1500 "Dayton"    & more
In reply to # 1502284 by CraigL Dan - you are saying if the brake warning light is on, you can center the PDWA shuttle switch directly to turn the light off rather than sightly bleed the front/rear brake systems to center the switch? I have no brake issue, but nice to know there is an easier way to center the switch on my 1975 Spit. Thanks, Craig

Yes you can, and it is fairly easy when you DO NOT have the center ball in there. That ball can be difficult to say the least. But, if the shuttle has moved, the ball will be loose, unless it has been that way for years and is corroded. You do have to be very cautious of leaks to the paint, and you will probably have to bleed the brakes afterward anyway. Doing it manually is usually just the way to get around having to do one side then the other because you press it too far and cause the shuttle to go to the other side.

Dan

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
I have in the past made a plug to replace the switch temporarily and hold the shuttle in place while I bleed the brakes.

As I stated earlier, if the shuttle moves, especally if it has not been activated for a long time, it will often begin to leak, as a result of the shuttle 'O' rings tearing on a crusty bore.

spits Avatar
spits paul krause
Sunnyandhot, FL, USA   USA
1968 Triumph Spitfire MkIII
1969 Triumph Spitfire MkIII "Rusty; Gone But Not Forgotten!"
1969 Triumph Spitfire MkIII
Jim
What model PDWA do you have?
As others have said, an off center PDWA shuttle alone won't adversely affect braking since there is no flow blockage. I'd be evaluating the system to ensure that it is configured per print and ensuring that the system is bled fully.
I wrote a white paper with input from Dan A specific to the early double plug unit that can be found with a forum search. I'm on a business trip or I'd attach it.



Best,

Paul



Edited 2 time(s). Last edit at 2017-12-11 02:15 PM by spits.

agoodcave Avatar
agoodcave Mike Cave
Oceanside, SoCal, USA   USA
Rimmer Bros now has rebuild kits. I tried removing the piston and could not get air out of the system to my liking.

I replaced the switch with a bolt and re-installed the piston. No leak, but I am going to get the rebuild kit and make it right.

Rimmer also sells a whole new assembly.

Just have to repaint the firewall around the PDWA and all will be right again...

spits Avatar
spits paul krause
Sunnyandhot, FL, USA   USA
1968 Triumph Spitfire MkIII
1969 Triumph Spitfire MkIII "Rusty; Gone But Not Forgotten!"
1969 Triumph Spitfire MkIII
In reply to # 1502321 by agoodcave Rimmer Bros now has rebuild kits. I tried removing the piston and could not get air out of the system to my liking.

I replaced the switch with a bolt and re-installed the piston. No leak, but I am going to get the rebuild kit and make it right.

Rimmer also sells a whole new assembly.

Just have to repaint the firewall around the PDWA and all will be right again...

Mike
Is this the kit?
That kit looks like it fits the PDWA pictured here.

I couldn't find, and have yet to find, a kit for the unit used in the Spits from 68 on, ie the double plug config. The double plug uses square cut O-rings which are all but impossible to find in the required size and material. I've rebuilt using round section rings as shown and it seems to be holding fine but the square are unicorns.



Best,

Paul


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Bpt70gt Avatar
Bpt70gt Brian T
Westmoreland, NH, USA   USA
That kit fits the TR6 and 250 but not the double ended PDWA in the Spits or GT6. A kit for those are unobtainium. Be careful using regular O-rings in the Spits and GT's, do the math and research. There are engineering standards for how the "squeeze" and bore finish conditions need to be. Carefully clean out the bore and use a 10x jewelers glass to inspect the bore. It will likely scare you. Anything but very good and pit free and you're gambling. I've opened up 10 of these and have yet to find a bore that is good enough even IF there was a kit.

spits Avatar
spits paul krause
Sunnyandhot, FL, USA   USA
1968 Triumph Spitfire MkIII
1969 Triumph Spitfire MkIII "Rusty; Gone But Not Forgotten!"
1969 Triumph Spitfire MkIII
Agree that most bores aren't worth working with. I have a couple in good enough shape to have rebuilt. Yes, you need to do your homework on the size and material.

The most common result of leakage here is corrosion in--due to hygroscopic DOT3--or leakage out of the switch cavity. Ultimately some spontaneous, catastrophic O-rings failure that allowed significant system to system leakage would result in a single hydraulic system comparable to that on all U.S. Spits up until '68; the UK cars didn't get a dual system until quite a bit later.



Best,

Paul

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