single line brakes to tandem

[dolly1850man]

my car is fitted with single line brake system , can i convert it with later tandem master cylinder and a bit of pipework , will it bolt to my servo or is there mods to be done

andy

[Carledo]

YES and YES!

Steve

[Richard the old one]

I have noted that the servo on my cars with the tandem brakes is of a larger diameter than my cars with the single line system, also the size, Diameter,of rear brake cylinders is not the same. Externally the brake cylinders are the same but if you check the pistons you will see that they are different.

[Carledo]

Having actually done this however I can say that the effect of these differences is unnoticeably small, probably zero.
Going Tandem does not in any real sense improve the brakes it just gives an improved safety margin insofar as if one circuit fails you still have one to fall back on (of course if that happens to be the rear one it will help to know some prayers!)
But if better brakes is what you seek, then spend your money on a Trackerjack conversion.

Steve

[Jon Tilson]

Utterly pointless.....

I have never had a single line master fail in service.....

All 3 duals have....

Jonners

[dolly1850man]

was thinking of the safety part of it , my brakes work perfectly but needs a couple of steel pipes replaced and thought would be good opportunity to do upgrades while i was at it but to what i gather regardless of single or dual master wont help if the rear brakes fail ??? i will just stick with the single master

thx all

andy

[Richard the old one]

I do not understand your comment "that a dual master cylinder will not help if the rear brakes fail."

You will still have the front brakes if the rear fails.

I consider the most important things to do are: 1. check your brake lines and hoses for deterioration & 2. monitor your hydraulic fluid level so you are aware if any seal starts to leak and can hence sort out the problem before it becomes a real problem.

[JPB]

....regardless of single or dual master wont help if the rear brakes fail......

The volume of fluid in the dual system's reservoir is such that, assuming the pads were about half worn, the fluid level could be low enough to escape before the shuttle in the PDWA could close off the rear circuit but it would most likely give you back some control with a couple of pumps at the pedal.
The advantage is minimal compared to, say, the dual circuit layout in a Volvo 140 series which gives you one rear caliper and two from four pistons in each front one per circuit so one of those can suffer a failure of quite a few pistons before the brakes would even become uneven in their effect, the Dolomite would be much less stable if the rear circuit were the only one left as is typical with a F/R split.

I've yet to encounter a Girling CV that failed randomly, they tend to give you some warning, but the dual circuit cylinder is more likely to die as Jon has suggested.

[Richard the old one]

The PDWA does not shut off the hydraulic lines if there is leak. On the tandem brake master cylinders that I have looked at the reservoir splits into two sections just below the low level line. If there is leak you will loose the hydraulic fluid from one section but there will be hydraulic fluid still in the other section.

[JPB]

The PDWA does not shut off the hydraulic lines if there is leak. On the tandem brake master cylinders that I have looked at the reservoir splits into two sections just below the low level line. If there is leak you will lose the hydraulic fluid from one section but there will be hydraulic fluid still in the other section.

There may well be, but only until the pedal is pressed! Damming the reservoir cannot prevent loss of pressure at the point where the breach occurred. Without that shuttle to close off the affected circuit, there'd be nothing to prevent the fluid from one circuit simply passing across the PDWA and escaping from the leaking part of the system.
All that would be in practical terms is a single circuit system with two pistons and two pipes entering the PDWA, no advantage over a simple Y piece in fact. There has to be a means of creating a solid pedal once a circuit has failed and that means is the shuttle valve making a seal in one end or the other of the PDWA. That is how it works.

[Richard the old one]

Has anyone found a technical description of the tandem braking system as I have not been able to find any handbook that details how the tandem system works but I have looked very carefully at the various parts of the system when I have stripped them down.

I agree that there will be a loss of pressure in the circuit which has the breach but this will only result in the loss of the hydraulic fluid to that half of the system as the master cylinder reservoir is split into two sections below the minimum hydraulic fluid level line.

The seals on the shuttle in the PDWA prevent the hydraulic fluid from the good circuit passing across the PDWA and escaping from the breach. If the PDWA did shut off the circuit when there was a leak it would not be possible to bleed the system.

[JPB]

.....The seals on the shuttle in the PDWA prevent the hydraulic fluid from the good circuit passing across the PDWA and escaping from the breach.

Absolutely correct, yes.

If the PDWA did shut off the circuit when there was a leak it would not be possible to bleed the system.

But, as you just said yourself, it does shut it off and the shuttle must be centred - after a leak has been repaired - before the system can be bled, in that sense the PDWA is entirely conventional.

[jikovron]

I note a late 90's volvo 900 item seems to fit any of the dolomite servos without modification and has 4 outlet ports for either 2 cylinder bleed nipples or 4 individual pipelines to each corner

i used it merely because it was easily available and had an outlet for the clutch hydraulics the relative question of whether it was more safe fell to its better quality

despite the 23mm bore the pedal pressure required didnt change noticably however with standard brakes i imagine it would fail brake efficiency with most testers being used to testing modern vehicles

[Carledo]

The PDWA is a complete waste of time and effort it doesn't tell you anything you need to know, goes off when its not faulty and makes the brakes difficult to bleed!
Anybody with an ounce of sense would do what i've done and throw it away!

Steve

If you do away with it you lose nothing cos the fluid can no longer cross it in case of a leak and you can use the wiring for a level sensing filler cap so you still get advance warning of fluid loss before it becomes catastrophic!

[Jon Tilson]

Funny that...
When I recently did the seals on my o -e dual master the red light was on, indicating a pressure differential due to a failing master seal.

Having replaced the master seals and bled it all it now doesnt come on....but I didnt bother centering anything in the pdwa. I guess I was just lucky.
Funny thing was that last time I did one I only had to bleed the front o-s caliper to get full pressure. This time I had to bleed air out at all 3 cylinder bleed points.

So I'd say it is useful if it does indeed work as intended and show a pressure differential. Of course how many cars it still works on is a moot point. Most ones I've seen have broken connectors or are just not connected at all. That being said, the excess pedal travel and the need for the odd extra pump but no apparent fluid loss told me far more than the warning light...

Jonners