The Triumph Dolomite Club - Discussion Forum

Hi everyone,

I wonder whether anyone can offer advice about the following brake bleeding snag on my 1980 1850HL. I've fitted a new tandem brake master cylinder (well a reconditioned one from the club) - the problem is I can't get fluid to bleed out of the rear bleed nipple.

Fluid flows out of both front calipers - but not a drop from the rear, despite plenty of pumping of the brake pedal...? Can anyone please shed any light on what may be causing this?

Also - as an aside - when pumping the brake pedal there's a load creaking/grating noise from the servo/master cylinder area - it doesn't sound right or healthy. Any diagnosis of that and the brake bleeding problem would be gratefully received. Thank you

With best regards

Stuck pwda. Dismantle, clean, refit and bleed.

Jeroen

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Classic Kabelboom Company. For all your wiring needs. http://www.classickabelboomcompany.com

And here's the counterpoint to the running question you asked in your other thread, you have screwed the servo yourself by pumping the brake pedal repeatedly to the floorboards in an attempt to bleed the rear brakes. This is a serious problem on all Dolomites but particularly on the slant powered cars. I'm not blaming you or calling you stupid as this is a peculiarly Dolomite specific problem and i'm not sure WHY it happens. But it does! I ALWAYS gravity bleed Dolomite brakes to avoid this problem, particularly on dual circuit cars where I bleed the fronts first (easier to do) to establish at least a quarter of a pedal before attempting the often more recalcitrant rears. Then if the rears DO require a pump or two to encourage them, I won't be able to floor the pedal, even if I tried and won't cause myself this problem and expense! If youda asked sooner, we'd have told you!

Jeroen is completely correct about the cause of your lack of bleeding success being the PDWA valve on the bulkhead, but my solution is a tad more radical than his, rip the stupid, inefficent and useless thing off the car and throw it in the bin. I can supply you with pipework and fittings to bypass it completely.

Steve

_________________
'73 2 door Toledo with Vauxhall Carlton 2.0 8v engine (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine (The Dolomega)
'72 Triumph 1500FWD in Slate Grey, Now with RWD and Carledo powertrain!

Maverick Triumph, Servicing, Repairs, Electrical, Recomissioning, MOT prep, Trackerjack brake fitting service.
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I know this will sound mad but could the problem be avoided by running engine? That way the servo would be assisting and you wouldn't be trying to force the servo to move when it has no vacuum on it? Just a thought.

Well Steve and I differ opinions about deleting or not as I don’t think it’s an akward device. In the drawing attached you can see the problem when not bleeding correctly.

ALWAYS start at the rear and when you are not sure your PWDA moves freely you best have the front depressurised first by opening the caliper bleed screws and leave these open. Starting at the rear has nothing to do with the conventional thoughts, longest line first in this case.

When all is depressurised because fitting new wheel cylinders or changing brake hoses and starting to bleed at the front the next thing happens. When the front system has it’s air out and is pressurized, the plunger moves towards the empty rear system blocking the rear inlet. Everytime you press the pedal the rear is blocked and the fluid can never travel any further towards the rear.

When all moves freely, at this point you realized you started wrong, you can depressurize the front again and start bleeding the rear. A tiny opening because the plunger moves back a little by the o-ring seal tension this is enough to move the plunger back again by fluid travel and opening the rear inlet totally and you can bleed the rears. The plunger will move towards the front system but never blocks it because of the construction.
Having the rears pressurized you can bleed the fronts and the plunger will stay in the middle and all works fine.

The thing is, there can be a lot of corrosion inside the PWDA. It does happen that when the wrong sequence is done, the plunger is stuck and blocking the rear inlet permanently. The the only option is to clean the PWDA so the plunger moves freely again and start bleeding the right way.

A blocked PWDA BECAUSE OF CORROSION WILL NEVER FREE ITSELF.

Jeroen

_________________
Classic Kabelboom Company. For all your wiring needs. http://www.classickabelboomcompany.com

Thanks for the responses guys. The servo was noisy before the master cylinder started leaking - I put it down to age - and now it seems, found out the hard way about the consequences of incorrect methodology with Dolomite brakes.

I'm favouring keeping things simple and removing the PDWA. So Steve, if you could let me know how much your PDWA retirement fitting kit is - and how to contact you directly to get that sorted, I'd be grateful.

And if you - or indeed anybody, has a spare servo waiting for it's next adventure - please contact me - I'm in need!

Thank you

I believe this problem is often caused by there being an air lock in the system. When it comes to bleeding the tandem system, I used to find this difficult some times, until someone on the clubs internet discussion board, recommended cracking the hydraulic pipe connections at the brake master cylinder, one at a time, while someone is pressing the brake pedal as air gets trapped in the master cylinder and is very difficult to get out unless you crack these connections and then retighten them once the air has stopped coming out and you see hydraulic fluid escaping. I can’t remember who provided this information now as it was a long time ago. I have also found that doing this improves the feel of the brakes as it ensures there is no air trapped in the master cylinder. There was no warning given that the brake pedal should not be pressed fully down as this would damage the internals of the brake servo but based on our current knowledge this precaution needs to be observed.

In the case we are considering as the problem is only in the rear brake line, I would be tempted to only initially slacken the rear brake line hydraulic pipe connection on the master cylinder. I believe this is the rear pipe connection to the master cylinder. Having done this I would then move on to slacken the rear hydraulic inlet pipe connection to the PDWA to check that the hydraulic fluid was getting to this point when the brake pedal was pressed. If this test is satisfactory and the connection has been re-tightened the next step is to slacken the rear hydraulic outlet pipe from the PDWA and to check again. If necessary you could work your way down the brake line until you find there is a blockage.

Please give it a try before you do anything else so that we are able to confirm what I believe could be the simple answer to the problem. You should still be able to bleed the system with your broken servo.

In respect to the PDWA I do not agree with soe8m (Jeroen’s) comment that the shuttle will move and block off the hydraulic supply to the rear brakes. This may be the case in some hydraulic brake systems but it is not in Dolomites. If a leak occurs in either the front or the rear part of the hydraulic system, the general level of the brake fluid in the Master brake cylinder will go down as the fluid escapes but when it reaches the minimum level mark on the reservoir, the reservoir is divided into two sections. See attached photo of Tandem brake master cylinder reservoir. The section connected to the leaking pipe work will continue to drain away but the other section will not and will work normally.

Some clarification questions for Carledo (Steve). I note you say you ALWAYS gravity bleed Dolomite brakes. Do you just open the three bleed nipples and then fill up the master brake cylinder and wait for hydraulic fluid to drip out of the bleed nipples? How long does it take? Do you just shut the front bleed nipples once the brake fluid has started to drip out and then wait for a bit longer before you, if necessary, pump the brake pedal a bit to force the brake fluid to the rear brakes but you ensure that you do not floor the pedal?

OK a few questions answered!

On gravity bleeding, my normal procedure for dual circuit cars (assuming a completely empty system) is to first fill and bleed the master from the unions on both circuits, yes, just wait till it starts to drip out, don't use the pedal. As the fluid starts to leak out watch for air bubbles, when they stop, nip up the unions. next, open both front caliper bleedscrews and wait again, it's seldom more than 5 minutes before fluid appears at each with the O/S normally first because the pipe run is shorter. Again, check for air bubbles and when the fluid comes out cleanly, close the screws. At this point, if you are brave and careful, you can check progress by gently and slowly pressing the pedal. It won't be a GOOD pedal, but if you've done it right, there should be resistance an inch or two off the floor. Checked and correct, you can then proceed to the rear, initially just open the screw and wait, up to 10 mins should be more than enough, if no fluid is apparent by then, administer 3 or 4 slow, gentle pumps, letting off again when you encounter the resistance of the front circuit and wait again. If you don't get fluid after that then time to suspect a problem with the PDWA.

Which end first? Since the 2 circuits are completely separate, and divided front and rear (rather than the modern diagonal split) it REALLY doesn't matter a pig's burp WHICH end you bleed first. You are bleeding 2 separate systems. The only reason I do the fronts first is that they normally bleed easier to give me the half pedal I like to save the servo.

PDWA, the reason I dislike the PDWA is that (besides making bleeding a PITA) it doesn't tell you anything you don't already know! PDWA stands for Pressure Differential Warning Assembly and what it does, is put a light on if it detects a disparity in pressure between the 2 circuits. Unfortunately, it ONLY does this when it's actually happened and AFTER the brakes are applied. And since you KNOW you have a problem because the pedal is on or close to the floorboards (!) it's a bit superfluous to have a light to illuminate your panic! So I ditch the thing and use the car's electrical circuit to run a master cylinder cap fitted float level sensor. This will warn of a dangerously low fluid level in EITHER circuit and, critically, will do so BEFORE you actually run out of brakes!

To the OP Dunhookin, I'm knackered and off to bed now, but i'll send you a PM with details of the PDWA bypass kit tomorrow night.

Steve

_________________
'73 2 door Toledo with Vauxhall Carlton 2.0 8v engine (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine (The Dolomega)
'72 Triumph 1500FWD in Slate Grey, Now with RWD and Carledo powertrain!

Maverick Triumph, Servicing, Repairs, Electrical, Recomissioning, MOT prep, Trackerjack brake fitting service.
Apprentice served Triumph Specialist for 50 years. PM for more info or quotes.

Hi Richard, i too bleed brakes by gravity as Steve does. I have just found it works and have never had a problem, probably comes of not having anyone around to press the pedal It does take a little longer but it does work. And when you do a pressure test for leaks just put a paper tissue around where you have been working and hold the pressure on.

Tony.

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NOW A CLUB MEMBER 2017057

I'd be reluctant to advise anyone to bin the PDWA. It's a very simple item to clean out and free off, takes minutes to do so. Also it's part of the braking system as designed. Remove it and you have modified the system away from it's type approved state. We are fairly liberal in this country, but in others you are simply not permitted to change things like this. Even here, technically a sharp insurance assessor could refuse a claim on the basis that a "safety system" has been bypassed. It's a stretch, I know, but in these times we live in its possible...

I alway’s put my Dolomite on my roof when bleeding the brakes…

There’s nothing mythical about Dolomite brakes or PWDA’s. 80% of the Brits had these in the ‘70s and 80’s and most of the USA cars also, physically exaclty the same thing what was fitted on the Dolomite, totally interchangeable, only some had a slightly different plunger type that cannot block lines in the extreme plunger outer.

Airlocks and undoing the lines on the brakemaster….

It’s just having fluid in the reservoir and pump the pedal and start at the rear. Like any other car.
Same as for he clutch. Fluid, pedal pump and bleed. Really nothing special.

And like Matt say’s, cleaning the PWDA is also nothing special. When the plunger does stick you can pry it out through the inlet and outlet holes, clean and refit. It will take longer to convert than to clean and refit.

When you that scared that brakemasters or PWDA’s fail by normal bleeding the brakes it’s time to check these parts before any bleeding efforts. Owners who don’t want to bleed by pumping the pedal affraid to damage a part or already know that a part will fail are driving ticking bombs already.

When the PWDA works fine the bleeding goes fine. When there’s no fluid to the rear the PWDA does block the line. It’s not designed to block but because of corrosion and residu’s and the design of the particular plunger it does block and not returning and opening the inlet to outlet of the rears again as is the problem of the topic starter. (unless a seal in the brakemaster is faulty)

Jeroen

_________________
Classic Kabelboom Company. For all your wiring needs. http://www.classickabelboomcompany.com

I would like to thank Carledo (Steve) for taking the time to provide us with detailed instructions on how to Gravity bleed the tandem brake system. I will certainly give it a go when I next have to undertake the task as it will prevent any possible damage being done to the servo. It should also ensure that if a reconditioned brake master cylinder is being fitted we are not misled into thinking that the unit is defective when the real problem is, more than likely an air lock.

I do agree with Steve’s comments on the PDWA but I would also add that I do not like the fact that the system has not been designed to be a fail-safe system. I have found that due to a lack of regular operation the associated switch contacts can corrode and so fail to make contact, when the switch is called on to operate. If the shuttle has moved from its central position and it can’t be reset in the mid position, the system will bring up the warning light but this can be overcome by removing the switch and then packing it out with one or two plain washers so that the switch’s plunger is not pressed in far enough to operate the switch. It is most unlikely that this bodge will be noticed and once done you do not have to go through the tedious procedure associated with setting up the PDWA to get the warning light out. I have previously stated that the PDWA shuttle does NOT shut off the defective brake line so it does not matter where the shuttle is within the PDWA. What I do recommend is regularly checking the master brake cylinder reservoir hydraulic fluid level or as Steve has suggested fitting a sensor to detect any drop in the hydraulic fluid level.

If you want to attempt to overhaul the PDWA, back in Dolly Mixture, edition 115 Spring 2006, John Abra, provided the details of what bits were needed and the procedure for doing the task. My recommendation is that if the PDWA is not leaking any hydraulic fluid into its central section, where the switch fits, I would leave it alone, as I consider, it can be difficult to get a seized shuttle out without damaging it and the internal bore of the PDWA.

I was just about to post this when I noticed that Jeroen had posted so now readers have the benefit of his knowledge to consider as well. I must have been lucky so far as I have not had a blocked line to the rear brakes.

I would like to thank Carledo (Steve) for taking the time to provide us with detailed instructions on how to Gravity bleed the tandem brake system. I will certainly give it a go when I next have to undertake the task as it will prevent any possible damage being done to the servo. It should also ensure that if a reconditioned brake master cylinder is being fitted we are not misled into thinking that the unit is defective when the real problem is, more than likely an air lock.

I do agree with Steve’s comments on the PDWA but I would also add that I do not like the fact that the system has not been designed to be a fail-safe system. I have found that due to a lack of regular operation the associated switch contacts can corrode and so fail to make contact, when the switch is called on to operate. If the shuttle has moved from its central position and it can’t be reset in the mid position, the system will bring up the warning light but this can be overcome by removing the switch and then packing it out with one or two plain washers so that the switch’s plunger is not pressed in far enough to operate the switch. It is most unlikely that this bodge will be noticed and once done you do not have to go through the tedious procedure associated with setting up the PDWA to get the warning light out. I have previously stated that the PDWA shuttle does NOT shut off the defective brake line so it does not matter where the shuttle is within the PDWA. What I do recommend is regularly checking the master brake cylinder reservoir hydraulic fluid level or as Steve has suggested fitting a sensor to detect any drop in the hydraulic fluid level.

If you want to attempt to overhaul the PDWA, back in Dolly Mixture, edition 115 Spring 2006, John Abra, provided the details of what bits were needed and the procedure for doing the task. My recommendation is that if the PDWA is not leaking any hydraulic fluid into its central section, where the switch fits, I would leave it alone, as I consider, it can be difficult to get a seized shuttle out without damaging it and the internal bore of the PDWA.

I was just about to post this when I noticed that Jeroen had posted so now readers have the benefit of his knowledge to consider as well. I must have been lucky so far as I have not had a blocked line to the rear brakes.

Just another thing i think is worth mentioning as its easy to overlook, bleed with the wheels on the floor (especially if a bias valve is fitted) and make sure handbrake is off.
It doesn't sound that's the case here though

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Some people are like Slinky's, they serve no real purpose in life but bring a smile to your face when you push them down the stairs.

A couple of final words on the PDWA before I let the subject go. Jeroen is perfectly correct in that these parts were almost universal on British cars fitted with Girling dual circuit brakes in the late 70s. Stags and TR6s and 7s had them as well as MGBs and Midgets and other stuff like (notably) MkI Cavaliers. But it's more than interesting to note that, after their introduction circa 1977, they were widely used, but then completely dropped, by 1982 they had completely disappeared from production cars. If they were that good, they'd still be using them today! IMO it was just a passing fad by Girling, Lockheed and other dedicated brake manufacturers didn't make or sell a comparable unit. Either that or they were an attempt to comply with projected legislation from somewhere (USA maybe?) that failed to materialize. Like the much touted "no more convertibles" law in California which caused all the early TR7s to be built as hardtops and which turned out to be BS.

The other point, and I know one instance is not a statistical universe, back in 78, I was road testing a brand new Cavalier 2.0 MkI after PDIing it and experienced a total brake failure which turned out to be a faulty PDWA. I wasn't going fast, fortunately and managed to haul it to a stop with the handbrake, but it gave me a bit of a moment! There was no fluid loss and i'm still not sure how I lost all pedal on a dual system, but there you are. It was replaced under warranty and, from the alacrity with which the claim was accepted, I infer it wasn't unheard of. OTOH, i've never experienced such a failure since, nor do I know anyone who has, but I have heard reports from other dual circuit Dolomite owners where failure of a single circuit has resulted in a total loss of effective pedal, which kinda negates the point of having dual circuit brakes! I don't know whether this is down to the PDWA, it's only a suspicion, none of the cars i've deleted the PDWA on (now in double figures) has yet suffered a circuit failure.

Steve

PS, Dunhookin, you have a PM!

_________________
'73 2 door Toledo with Vauxhall Carlton 2.0 8v engine (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine (The Dolomega)
'72 Triumph 1500FWD in Slate Grey, Now with RWD and Carledo powertrain!

Maverick Triumph, Servicing, Repairs, Electrical, Recomissioning, MOT prep, Trackerjack brake fitting service.
Apprentice served Triumph Specialist for 50 years. PM for more info or quotes.