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Brake part problem - What is this DPWA?


Ciaran
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I've got a problem identifying a part - and boy could I ever use some help from the smart folks on this forum

I have a series 3 hybrid. Er... very much a hybrid. On the braking side, its got a series III (LWB) master cylinder (2 ports output) - coupled with Range Rover classic Axels and caliper brakes (front and back). The two brake fluid pipes from the master cylinder connect to a differential pressure warning actuator (DPWA) which has three output ports (and an electrical switch which activates if there is a significant pressure imbalance between the front and rear circuit). Out of the three ports, two connect to the front brakes, and one connects to the rear brakes.

The DPWA block has failed. Internally the seals have broken down, and fluid at pressure is leaking out of the electrical switch. No amount of sealing cures the problem. See attached image of the failed part.

Ok - so I looked at the land rover parts book for Series III, and figured that a suitable replacement would be an NRC4880, only to find when it arrived today that NRC4880 has just four brake fluid ports and not five. The one on the front shown as output 1 of 2 in my picture is missing.

There are no markings on the failed unit itself other than "Girling" and the number "0006" on the reverse side - which incidentally are also on the wrong part NRC4880.

I'm stuck to figure out what part to order ?

I know these parts have a bad reputation (they make bleeding... tricky) but ideally I'd like to replace it with the working (albeit now faulty) part.

Has anyone else experienced problems with this part - and if so, how did they approach a fix for it ?

Any comments very gratefully received.

Ciaran

post-23531-041791900 1291127698_thumb.jpg

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on a 110 prior to the brake rationalisation, this was the brake pipe layout

page 494 the front brakes have 1 feed pipe which then splits at a T piece one coonection goes to the right front flexi hose, the other connection carries on with rigid brake pipe to the left front flexi hose, the 3rd connection is from the PDWA unit, so the PDWA only needs 4 pipe connections

1.front brakes

2.rear brakes

3 & 4. Master cylinder input to front & rear pistons of PDWA

so you can connect as for a 110 brake pipe layout.

HTH.

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Fisrtly: It's a switch only, it plays no part in isolating one part of the system in case the other fails. It can safely be omitted, but you'll lose the warning light facility. PDWA = Pressure Differential Warning Aparratus.

The missing port connects directly with the other two adjacent ones. IIRC it is used for the front brakes, you can use the 4-port item with a T-piece in the front pipes.

I had a LWB Ex-MOD SIII which a 4-port switch and a T-piece to split the front pipes to the left & right.

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My 109 uses a single line from the PDWA to the front cross member, where a T-piece splits it to each corner.

I will be fitting the fluid level sensing cap from a Discovery to my reservoir - this will achieve the same thing by warning of a leak, hopeful before the leaking circuit loses enough fluid to move the DPWA piston.

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............

I will be fitting the fluid level sensing cap from a Discovery to my reservoir - this will achieve the same thing by warning of a leak, hopeful before the leaking circuit loses enough fluid to move the DPWA piston.

I think the PDWA would be quicker for a sudden leak.

Also the master cylinder cuts off the leaking side when a fault occurs so the fluid level in the resevoir may not drop enough to close the switch.

There is also a facility to fit a switch in the top of the pedal box, this will close when pedal travel becomes excessive, thus warning of air in the system or brake wear and the need for shoe adjustment or replacement.

Edited by rtbarton
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There are 2 seals in the PDWA - which if I recall correctly from my Triumph Stag were in fact simple 'O' rings. I was able to buy a repair kit for that - seem to remember it came from BL rather than a brake supplier.

As has been said the later vehicles have 2 outlets only - with the front one going to a 'T' piece.

Do you know what size the ball-bearing is under the switch? There is apparently supposed to be one which had disappeared on my S3.

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Also the master cylinder cuts off the leaking side when a fault occurs so the fluid level in the resevoir may not drop enough to close the switch.

The master cylinder isolates one circuit from the other, but you still loose the fluid from the faulty circuit which lowers the whole resevoir level by approx 50%, hence the level switch in the cap.

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The master cylinder isolates one circuit from the other, but you still loose the fluid from the faulty circuit which lowers the whole resevoir level by approx 50%, hence the level switch in the cap.

The piston on the faulty side blocks fluid flow as it moves further than normal, there may be a small amount leaking, but the PDWA will switch before the resevoir switch does.

The resevoir switch detects fluid loss beyond the minimum level, so if the resevoir is full when the fault occurs the level may not drop enough to operate the switch.

I've put some diagrams in my album:

Album

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You'll have found out anyway as the PDWA won't operate on a slight leakage or weep - and so will only tell you what you've just found out as your foot sinks to the floor.

I don't think it actually restricts the fluid flow to the damaged circuit - I can see this as being far more dangerous that a leaky wheel cylinder which still functions reasonably well.

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You'll have found out anyway as the PDWA won't operate on a slight leakage or weep - and so will only tell you what you've just found out as your foot sinks to the floor.

I don't think it actually restricts the fluid flow to the damaged circuit - I can see this as being far more dangerous that a leaky wheel cylinder which still functions reasonably well.

Your foot won't sink to the floor unless both circuits fail, the piston (in the master cylinder) in the faulty circuit will limit before that happens. Have a look at the pics I mentioned above.

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I appreciate that in the event of one circuit failing the pedal shouldn't hit the floor - but when it did happen to me there was a very considerable increase in travel and the pedal was very close. (back brake on a Renault 21 stuck and boiled the fluid I think - as after pouring water over the wheel and leaving it for a bit all went back to normal apparently)

My personal opinion is the the PDWA is an expensive and quite useless complication - which curiously doesn't seem to be fitted to foreign cars. (The Renault 21 didn't have one nor have my Citroen ZX's. My BX was rather different - having decent fully powered brakes, probably spoiled by ABS)

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Its interesting that this shuttle valve does seem to cause rather strong opinions - perhaps due to the arguably needless inconvenience when bleeding the brakes. I have by now confirmed that the 5 port device I needed had part number 599443 - and within an hour of visiting Dunsfold I had one fitted to the landy, and was setting about bleeding the brakes. Got to say, that the bleeding was as simple as could be - all it took was a firm but slow moving foot on the pedal and the new shuttle valve didn't budge at all.

Before my venture into the wonderful 80W90 world of Landys - my background was with american cars, mainly those dated round the tail end of the 60's and early 70's. I have yet to see one american mass produced car that DOESN'T have a shuttle valve (equivalent) fitted. They all do. The only one thing that GM added to all their valve parts was a nose cone push rod which effectively locks the shuttle into a fixed position to make bleeding the hydraulics easier. It may be mandated by federal law, but someone out there obviously thinks they are rather a good thing.

One other respondent above (and thanks to you all - your comments were very welcome) asked about a ball bearing - which the haynes manual mentions. I'm sorry this won't help you - but I did strip the switch out of the new 599443 part before fitting it in order to measure that ball bearing size for you - but found that there actually is no ball bearing in the design of this part. Obviously the BB was only fitted to a different shuttle valve part.

Thanks again all

This is a superb source of knowledge

Ciaran

I appreciate that in the event of one circuit failing the pedal shouldn't hit the floor - but when it did happen to me there was a very considerable increase in travel and the pedal was very close. (back brake on a Renault 21 stuck and boiled the fluid I think - as after pouring water over the wheel and leaving it for a bit all went back to normal apparently)

My personal opinion is the the PDWA is an expensive and quite useless complication - which curiously doesn't seem to be fitted to foreign cars. (The Renault 21 didn't have one nor have my Citroen ZX's. My BX was rather different - having decent fully powered brakes, probably spoiled by ABS)

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Thanks for your answer concerning the ball bearing. I had to replace the switch on a PDWA and there was no ball bearing there. My early S3 workshop manual referred to the ball - so I thought it may have been lost before I got there!

Wonderful things - make 2 separate circuits and join them with something that can leak!

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