TR6 Source for TR6 pressure switch [pdwa] rebuild kit?

Dear all,

Tonight I found my brake master cylinder reservoir was low, and after a little bit of looking I surmised there was brake fluid coming out of the pdwa switch. It is an early car - 1971. Does anyone know who can supply a rebuild kit for the actuator?

Thanks a lot! Pete

If it's leaking at the PDWA, it's either:
1 - one or more of the 4 brake lines that need to be tightened.
2 - the fiber washer at the end of the PDWA,
3 - the fiber washer under the switch, or
4 - through the switch itself (unlikely) or it might be loose.

Internally, there are two rubber o-rings, but they won't leak outside of the housing.

The fiber washers are easily replaced.

Try to locate the source of the leak but AFAIK, there is no kit available that contains the washers as they are a common item. The switch is available from the big 3.

It is more common that the reason for a low fluid reservoir on a 6 is an internal leak between the m/c and the servo diaphram. But since you say that you see an external leak, check the above.

You may also want to check this out if you haven't seen it already. Good overview.

https://www.zen11896.zen.co.uk/vixen/parts/pdwa.html

I don't know the change point offhand, but Nelson Riedel wrote that there was an early and late PDWA used on the TR6. Early seals are NLA, but the later units take ordinary EPDM O-rings. He made a replacement piston for his PDWA.

PM me your email addy, if you would like a copy of the article; or perhaps someone can post a link.

PeterK said:

If it's leaking at the PDWA, it's either:
1 - one or more of the 4 brake lines that need to be tightened.
2 - the fiber washer at the end of the PDWA,
3 - the fiber washer under the switch, or
4 - through the switch itself (unlikely) or it might be loose.

Internally, there are two rubber o-rings, but they won't leak outside of the housing.

Click to expand...

Don't you think that if one of the internal o-ring's goes, fluid is going to make its way out of the housing via the switch? The switch socket is basically filled up with brake fluid. The master cylinder is pretty new. In my experience, when that goes some of the fluid makes it into the driver's footwell.

Yes, the switch does not make a seal, and could never hold up against the hydraulic pressure if the internal o-rings go.

I replaced the whole unit on my TR250 with a new later model version. Not cheap. Even Apple would not do a rebuild. Alternatively you could just plug the switch hole and run without it. This unit is intended to signal a failure in one of the break circuits, but you should notice this through the pedal before the idiot light would tell you.

PeterK said:

If it's leaking at the PDWA, it's either:
1 - one or more of the 4 brake lines that need to be tightened.
2 - the fiber washer at the end of the PDWA,
3 - the fiber washer under the switch, or
4 - through the switch itself (unlikely) or it might be loose.

Click to expand...

I had a leak that was through the switch. One of the o-rings gave out, and out came the brake fluid. Re-paint time.

Here is the link to the Wiki Triumph page that will take you to the Buckeye article previously referenced.

https://www.britishcarforum.com/tikiwiki/tiki-index.php?page=Triumph

Has/is there been a definitive answer about these valve being just a safety valve or do they act as a proportioning valve also?---While Im waiting for a new switch to come in, I pulled the inside valve out and have been using the brakes that way and I seem to have more solid and even braking. But I still lean towards it also being a proportioning valve, because without the insides both front and rear of the Master are going to both front and rear brakes. What are some thoughts on this?

I don't believe the PDWA switch does any proportioning. However you do not want to run it without the insides - you now only have a single-circuit braking system. Better to bypass the PDWA and just run seperate front/rear circuits.

Rob.

The PDWA does not impede the flow of fluid to either circuit, even when the piston is all the way to one side or the other. Thus it cannot act as a proportioning valve.

It could potentially act to some extent to equalize the pressure between the two sides, but I don't believe this happens in practice.

Re: Source for TR6 pressure switch [pdwa] rebuild

ALLAN said:

Has/is there been a definitive answer about these valve being just a safety valve or do they act as a proportioning valve also?...

Click to expand...

Perhaps this will help; it's something I seem to end up posting various places every couple of years, and this is from a June 2002 posting to the Team.Net Triumphs list:

Pressure Differential Warning Actuator

"The P.D.W.A. is an 'inline' hydraulic valve through which both brake fluid lines are routed. The purpose of the device is to detect failure in either of the systems and to transmit, electrically, warning of the failure to a light on the facia."Triumph GT6/Vitesse 2 Litre Workshop Manual, p. 3.230.

Master Cylinder

"Application of pressure on the push rod moves the primary plunger up the cylinder bore and allows a spring loaded tipping valve to return to centre. The primary supply port is closed by the valve and further movement of the primary plunger results in hydraulic pressure being transmitted to the wheel cylinders of the front brakes. At the same time the pressure created acts in conjunction with the increasing force of the intermediate spring to overcome the stronger secondary spring, thus actuating the secondary plunger.

"Initial movement of the secondary plunger closes off the centre valve supply port and the hydraulic pressure is transmitted to the wheel cylinders of the rear brakes.

"In case of failure of either chamber or circuit, mechanical contact takes place within the cylinders and the remaining chamber builds up the normal pressure to operate the brakes that it controls." same manual, p. 3.226.

--

Any questions? /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

It is a "warning actuator," thus it's purpose is to turn on the brake warning light if it detects a pressure differential change when one side of the circuit fails. As the inner valve moves to actuate the light, the failing circuit is blocked off so you retain the remaining circuit for braking. But it does not affect the proportioning of the front to rear balance, just turns on the light and the o-ring seals off the bad side.

Re: Source for TR6 pressure switch [pdwa] rebuild

All you guys are probably right and I should not be running without that valve, but it will go back in when I get the new switch. I got my old spare PDWA and I notice that the front plug goes in and restricts the front line opening by 50%--does that not do proportioning some how?---Rob I dont think you can run without the switch because that is what keeps the valve centered -- (except when there is a problem and the switch gets activated) this is why I took my valve out because I removed my bad switch and pluged the hole.

ALLAN said:

Rob I dont think you can run without the switch because that is what keeps the valve centered -- (except when there is a problem and the switch gets activated) this is why I took my valve out because I removed my bad switch and pluged the hole.

Click to expand...

The brake pressure being equal in the front and rear circuits will keep the valve centered - if all is working correctly the master cylinder will produce equal preasure to the two circuits as it "shares" the same piston. The PDWA and switch just detects when the pressure is not equal (the unequal pressure pushes the PDWA piston one way or the other).

So if you take out the PDWA piston, instead of there being two seperate circuits for the front and rear brakes, they will now share the same fluid through the empty PDWA housing, and a failure of the rear circuit will cause there to be pressure loss to the front and visa-versa. One fault in the system and all your brakes are gone.

If you take out the entire PDWA, and connect the brake line from the front of the M/C directly to the lines for the rear brakes, and the line from the rear of the M/C directly to the front brakes, (you should just need two couplings in place of the PDWA) you will still have two seperate break circuits, and so a failure of one circuit will still leave you with either front or rear brakes. You just don't have the idiot light any more.

Hope that helps,

Rob.

Rob, that is a good explanation but I still dont think that valve will stay centered without the switch in place, any small fluctuation in pressure from the master circuits will eventually shift the valve to one side or the other and then there will be a failure as you explained.---------still the one question, what about that front plug that restricts 50% of the front line opening?, proportioning or not?

This article does a great job describing the braking system in including the PDWA;

https://web.archive.org/web/20040229065356/www.buckeyetriumphs.org/technical/Brakes/Theory/Theory.htm

PeterK said:

As the inner valve moves to actuate the light, the failing circuit is blocked off so you retain the remaining circuit for braking. But it does not affect the proportioning of the front to rear balance, just turns on the light and the o-ring seals off the bad side.

Click to expand...

Actually, it doesn't even seal off the bad side. That tapered nose on the piston still allows fluid to flow into the side with less pressure (if the MC can still produce fluid). Thus, even if your rear brakes are badly out of adjustment (which can activate the PDWA), they still get pressure eventually.

Which is a good thing, since there are a lot of cars running around with the PDWA piston at one end and the wire just disconnected.

ALLAN said:

still the one question, what about that front plug that restricts 50% of the front line opening?, proportioning or not?

Click to expand...

Not. There is no proportioning on the dual circuits. If all is running correctly they are at the same pressure. The shared piston in the M/C makes sure of this. When you put your foot on the break pedal, you are compressing the break fluid going to the front and rear circuits the same amount. If you had two seperate M/Cs for the front and rear connected to the break pedal (as some set-ups do - mostly for racing etc)then you need a proportioning valve of some type so that you can adjust the pressure to the different circuits. For race cars, this can even be adjusted during the race, as track conditions change.

Rob.

Really good discussion guys!

I've toyed with the idea of using a dial type proportioning valve on the rear circuit and eliminating the PDWA on my 4A(converted to a TR6 m/c with servo and PDWA). That way I can add more rear bias on tight autocross courses to add a little rear steer with a punch of the pedal and twist of the steering.

We used to have a Formula 500 with a setup like that and it was a lot of fun on 180 degree turns! Of course, the F500 stayed nicely planted at the rear end coming out of the corners as long as you kept your foot into the throttle.

Re: Source for TR6 pressure switch [pdwa] rebuild

RobT said:

ALLAN said:

still the one question, what about that front plug that restricts 50% of the front line opening?, proportioning or not?

Click to expand...

When you put your foot on the break pedal, you are compressing the break fluid going to the front and rear circuits the same amount.

Rob.

Click to expand...

Same pressure but with a restriction less volume which might have some effect on the proportioning?? (not trying to be argumentative, im just not completely sold on no proportioning)

----------I just got my switch in the mail from Moss, interestingly it is in a Ford package with Ford part number. I looked it up, it fits Mustangs 67-69 and some other Fords and Mercurys 67-72. Part number C8AZ-2B264-A it sells on the internet for $20. and is also supposed to be available at NAPA. I will put this info on a new post.