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Triumph TR6 - PDWA Valve - Brake Problem

A friend with a very nice BRG '70 TR6 just had a local shop (experienced with Brit cars) install his second new brake master cylinder. The MC was bench-bled, installed and the system bled as per normal. Firm brake pedal but driving down the road, hit the brake and it travels almost to the floor. The problem comes and goes. They thought the first new MC was faulty and returned it for this one. There are no leaks at wheel cylinders or calipers and all unions and brake lines are sound. I'm thinking PDWA valve. His '70 does not have an electrical switch - has a bolt where the switch should be but it's been like this 8 years with no problem. I couldn't find anything in CDII for this problem. He has borrowed a spare PDWA from a buddy but a new one is about $190.00. Is there a rebuild kit or does anybody have any idea what his problem is?

Bob Evans

pretty much the worst that the PDWA could do would be to leak fluid due to bad seals. There is no "official" rebuild kit available, but the seals needed are pretty common, more on later. A few questions are in order. Is there a loss of fluid despite no leaks at any of the system joints or other braking cylinders? If there is no loss of fluid, what kind of shape are his brake discs, front hub bearings and/or stub axles in? If the brakes have alot of warpage or if the front bearings are loose/heavily worn or if there is excessive flex in the front stub axles, the pads can get knocked back in the calipers and it takes a pump or two of the brake pedal to move them back out to where the brakes can properly function. If one of these things happen to be the problem, until the pads are pushed back out, you can have a very long (as in to the floor) pedal. The solution is to address the cause be it discs, bearings or stub axle.

O-rings to rebuild the PDWA can be purchased from your friendly local rubber supply, places like McMaster-Carr, etc. The O-ring is a -008 size as defined by AS 568 (an SAE publication, possibly available from a database service at your local library if you want to see it), the important part is to get the size and the material right for the application. The O-ring is a nominal 3/16" ID, 1/16" section diameter, which in turn would make the OD 5/16". The material you to to use is Ehthylene Propylene also refered to as EPDM. To rebuild, remove the PDWA from the vehicle, remove the switch (or in your friend case, the bolt) then run bolts of the proper thread and diameter into the front and just one of the rear circuit ports (don't run them down tight as you can damage the sealing surface). Then remove the large nut on the end of the PDWA. Aim the open end of the PDWA over a container to catch the piston as it comes flying out. At the open brake line prot, apply a blow gun from your compressed air system and that piston should come flying out into your catch container. Every now and then the piston gets stuck in there. I've never had than happen on my end, but seem to recall Doug Baker having a probelm with that. I can't remember if he went with use of a penetrating solution or with application of heat to help free things up. Perhaps he will chime in on this.

Hope this helps you out.

You have a good memory. My PDWA issue is still unresolved. I have the new O rings you sent, but still have not gotten the piston out. I was a bit reluctant to use Aerokroil. I have not had a good air application so decided to wait until I could get the proper size bolts to plug the holes and a proper air connection. You may recall that it took over 100 psi to get my brake pistons out so the fact that the car sat unused for about 18 years has contributed to many surprises. I'm all done with the brake rebuild except for new lines that I'll install when the chassis is finished, and of course, getting the PDWA with new O rings!!

Bob, Steve's advise is sage. Good luck to your friend. I'd test the brakes again and see if you get more pedal with pumping a couple of times. If so, it's likely one of the things Steve has described, maybe a warped disc.
Doug Baker

What an excellent and generous response Steve - congratulations.

It is great to see such a detailed and knowledgeable reply. I agree entirely with your first paragraph and have learnt something from the second one.

To see such unsolicited and considerate help says a lot for this whole board and it's many contributors and seeing it makes me feel good -thanks.

And y'all have a good day too!!!
Roger H

I am unable to help on the valve problem but I would like to thoroughly endorse Roger's comments on both the quality and quantity of the responses provided by Steve and Doug.

This is surely what owing these cars and being on this particular board is all about.

Peter Thomas

Roger, Steve,

I agree... We have great helpful members on this BB.
Steve and Doug have helped me out with many things. Charlie Ballard helped me get my carbs adjusted, Both of you chime in with good advice regularly, and Rick Crawford was able to set the record straight for me that Elsinore is not a real beer (actually he's been very helpful as well..remember his ZDDP reasearch?). Those are just a few of the people that regularly contribute.

Now that we have the upload feature the possibilities for help are endless. 'A picture is worth a thousand words'. I'm almost looking forward to another problem to solve....forget I said that.

HP Henry Patterson

One thread we are yelling at each other and the next we are patting each other on the back.

This thread is more representative to the attitude on this BBS.

Yes...good reply Steve.

If I may add a few probably is not pads as you would see worn pads (at least on the front) when bleeding the brakes. I would think warped rotors but then you would have felt the tipical peddle pulsing as brakes where applied. A quick lift of one front tyre off the ground and a wiggle of the tyre will show any play in the bearing area.
Finally, when done you should re-center the PDWA plunger so you have correct fluid flow front and back.
Rick Crawford

Thanks Steve. I'll print this and pass it on to Bruce to give to the shop guys. Excellent advice and a lot more to consider than just leaks, etc. About time he spent some $$ on the '70 anyway!

Bob Evans

I just went through a somewhat similar situation in that I recently awoke a 6 after a 15 year slumber. I rebuilt all the hydraulics and was still having problems with sticking calipers. I rebuilt the PDWA switch which still didn't fix the problem. Turns out the rubber hoses from the hard lines to the calipers where collapsing on the inside which allowed one way pressure into the calipers but bled it very slowly out. I bought the PDWA rebuild kit from British Parts NorthWest.
RC Chesavage

Thanks Bob for the inquiry on my behalf. The problem is now fixed. After installing a loaner from a friend, we took the old one apart and there was nothing inside the valve!! So now I have to get a new one or hopefully find a good used replacement. Thanks again Bob.
Bruce Sheppard

A gutted PDWA defintely changes the equation, didn't think of that one. Must of been that OZ economy stimulation I had been involved in earlier that evening by partaking of "a few" glasses of their wine at a neighborhood cookout.

As for the PDWA rebuild kit, good to know that someone has finally decided to sell one. Going the O-ring purchase route, it was sometimes difficult to be able to buy only the quantity that you needed as they would often not make the minimum sale amount or they were in packages that were not sold broken. There's probably more than a few people out there that bought a bag of 50 or 100 O-rings at $4 to $10/bag to get the two o-rings that they needed to rebuild the PDWA. Somewhere buried deep in one of my boxes of junk is a bag with 90 some odd O-rings for the PDWA.

As you can see gents, friend Bruce's problem has been solved - no guts in the PDWA - who'd have guessed? Now all he has to do is find buddy Steve a relpacement for his loaner. Thanks for the help.

Bob Evans

This thread was discussed between 28/05/2007 and 31/05/2007

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