> Alan, I'd go with the original type cylinder and equipment in as new
> condition, and with regular checks. The extra maintenance overhead on
> a dual system hardly seems worth it. If you did decide to dual you
> could use diagonal layout but you would still need some sort of warning
> system if one half went down. Could also depend on how many miles you
> want to do, whether you tow a loaded trailer, etc. Gordon, Falkirk,
> Scotland. 8-)
Hi, Gordon,
When you mentioned a 'diagonal' system, were you referring to a "dual triangle" system
like Volvo uses? If I recall, in Volvo's system, each front caliper has two separate
pistons, hence there are two brake lines going to it. The rear brakes have one line each.
Therefore, one circuit consists of both front wheels and the left rear wheel, while the
other circuit consists of both front wheels and the right rear wheel. It's a good system,
since each circuit gives you full front-wheel braking power, but I'm afraid that it would
involve some very complex modifications that I don't want to get into.
I'm just talking about doing a typical front-rear split like many newer vehicles use, with
a warning light to indicate if either circuit has failed.
As to maintenance, a dual-triangle system like Volvo's would have quite a few more parts,
but a typical all-drum-brake, front-rear-split system only has a couple of extra parts in
the master cylinder, plus a warning light switch. (Even in systems that happen to include
a proportioning valve, it seems that this valve should be a simple, no-maintenance item.)
Therefore, I personally can't see where there would be any significant increase in
maintenance or reduction in reliability. On the contrary, I would think that the chance of
a total brake loss should diminish considerably.
To my knowledge, all US motor vehicles and vehicles imported to the US from about 1967
onward had dual master cylinders and if anything, it seems to me that the number of total
brake failures has gone way down, rather than up. Perhaps one can find data related to
this topic in the archives of the NTSB (National Transportation Safety Board) or perhaps
the NHTSA (National Highway Traffic Safety Administration) or NCSA (National Center for
Statistics and Analysis). And, in case anybody wonders, no, I have NOT looked up this
data, so please, everyone, don't quote me as to the safety of dual master cylinders. It's
only my own perception that they're safer.
The only 'gotcha' that I can think of is that you have to take extra care to bleed a dual
system correctly, especially when installing a "dry" dual master cylinder. I like to
pre-bleed the master cylinder on the bench with short hoses between the outlets and the
reservoir until all bubbles disappear from both. Then I finish the bleeding process in the
standard manner after hooking it up to the system. I've found that this approach saves a
lot of messing about.
Your point about a warning system is well taken and I think that there is already a simple
solution for that. You install a differential-pressure-operated switch in the brake
circuit that is closed when one circuit (i.e., front vs. rear) gets considerably more
pressure than the other. This switch turns on a warning light and the switch remains in a
closed position until it is manually reset. This type of switch was used specifically for
this purpose on a bazillion vehicles and I wouldn't think that it would affect the
reliability of the system.
Even without a warning light, I think anyone would notice immediately if the front half of
the system went bad, and I think that a lot of people would notice if the rear system went
bad, at least if they had a long-travel brake pedal linkage and no power assist. On the
other hand, I've driven some cars that had naturally mushy power brake systems with too
much power assist and very short brake pedal travel...on those cars you might not notice a
rear brake problem unless a warning light came on.
With all the discussion lately on "pure restoration vs. aftermarket" etc., I wanted to
mention that since I plan to drive this vehicle on a fairly regular basis, I wouldn't feel
one bit guilty about making a modification such as this if it enhances its safety.
Besides, one could always convert it back to the old single master cylinder in a few
hours, using a couple of pieces of NOS brakeline.
I prefer authentic restorations whenever practical, but I also have great respect for
innovation and imagination. Every situation is different. In this particular case, safety
will probably win out over absolute purity.
Final note: I still have more homework to do before I determine to my own satisfaction
that I want to go ahead with the dual master cylinder conversion. I'm still open to
suggestions.
Thanks,
Alan
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