dual master cylinders?

[nostatic]

Which 911s (if any) have dual master cylinders? From the behavior of my SC when my brake line got ripped off, it would seem that is was a single cylinder. Is an upgrade to dual possible?

[jluetjen]

Sure. But they're not cheap. Smart Racing has a really nice Fabcar piece that will bolt into later model (ie. Vacuum assisted) 911's. I had one briefly and ended up selling it since I didn't want to cut my early car to make it fit. According to their catalog they sell them for $750.

For street use there is really no reason. I never touch mine. I only put in a dual master cylinder since my original master cylinder was bad and my car's ultimate use will be as a race car. So I figured I'd spend the money now rather then buy a stock master cylinder which I'd end up throwing away again after 2 or 3 years use. The big red adjuster on the dash has a great bling factor, but like I said, now that I've set it, I haven't touched it in over a year.

[Early_S_Man]

All RSRs and the '84 SC/RS had dual master cylinders with adjustable bias bars from the factory ... and those are the hardware 'imitated' by update kits from Smart Racing and other suppliers!

[Hugh R]

Wait a minute:

Are you saying that my 84 ROW doesn't have a dual master cylinder (two circuits). As in if I lost one caliper, I lose the brakes for the entire car? My DB4 has one braking system, and I lost a line once and the pedal went right to the floor, when I drive it, I try to never forget that.

[Tim Walsh]

Hugh,
There's a difference between a dual curcuit master cylinder (what you have) and a dual master cylinder. The dual curcuit is 1 cylinder but has 2 independent valves for the brakes. The dual master cylinders is two seperate assemblies connected by a bar. BIG difference.

[nostatic]

Originally posted by jluetjen

For street use there is really no reason.

well, after having a front line rip off, then zero brakes doing 65, I'm not sure I agree there is *no* reason for it. I wasn't thinking anthing fancy...don't all cars now run dual master cylinders? Forgive my ignorance on this...

[Hugh R]

I believe the U.S. DOT started requiring dual braking systems in the early 1960s, I believe some Volvos, and other cars have three so that if you rip a break line off of one wheel the other three still work. The DB4GT Aston Martin had two single valve master cylinder connected by a bar as well.

[jazzbass]

Originally posted by nostatic
well, after having a front line rip off, then zero brakes doing 65, I'm not sure I agree there is *no* reason for it.

Isn't this why you have a cable-based emergency brake? Just don't yank up to hard, or you'll find out how easily your car spins .

[nostatic]

ahh...well does an SC have a dual-circuit system?

[jluetjen]

Yes, most cars have duel systems (diagonally - RF-LR, and LF-RR), but generally one master cylinder. When you're talking about dual master cylinders, you're usually talking about 2 physical cylinders, one for the front and one for the rear.

In the situation that you described with losing one circuit at 60 mph, you'd really want to have the diagonal set-up since it is a lot more stable with one circuit out. With a front and rear circuit, if you lose the front (which is the first to snare on anything), the result will be liking pulling up the parking break.

[Bill Verburg]

All 911s since the late 60's have a dual circuit m/c. This is accomplished by using 2 separate coaxial pistons in a single bore as pictured here

On 911 one piston services both front calipers and the other services both rear calipers

Other Porsche models use a similar setup w/ stepped bores. i.e. the 2 m/c pistons have different sizes but are still coaxial

Many race cars use a dual circuit design but w/ dual master cylinders. here is a pic of a Tilton dual master cylinder setup

this setup has several advantages such as the ability to alter bias on track.

[na2ub]

I figured that all reasonably "modern" cars had the dual circuits, but both NoStatic and friend of mine had zero braking ability after losing a brake line. It would seem that the "system" does not work ... no?

[Bill Verburg]

Ones ability to make valid observations in such situations is questionable.

If only 1 front line was ripped off and the system was previously nominal, then the remaning subsytem will remain nominal.

Do not forget that the rears do relatively little in the way of stopping the car any way(as long as the car isn't arse backward

[nostatic]

Thanks for the info Bill (again). I know all about rear brake ineffectiveness...try bombing down a fireroad on a mtb and not using your front brake

I will consent that my observations might have been a little "off" during the incident. First the "blam", then the "hmm, tires flat", then the "hey, brake pedal goes to the floor" tends to remove objectivity. I think the train of thought is something like "crap, crap, crap, crap, crap".

It might have been if I'd pumped the pedal I would have gotten something. I've got that ingrained from my days of my '67 bug that had marginal brake systems, but in this case, I didn't really need to panic stop...I could just coast to the right, then ebrake.

The diagonal system at first blush seems to make more sense since you've got some front braking happening, but that might also set the car up for some bizzare handling.

[Early_S_Man]

The dual-circuit brake systems came to the USA market for the 1967 models ...

The vast majority now( and for at least the first 10-15 years of all production models) are split front-rear, not dual-diagonal, as pioneered by Volvo or Saab -- I don't remember which! The reliability of the front/rear split dual-circuit brake systems has been so good that most car mfr's don't see the benefit/cost ratio as justifying the extra cost and complexity of dual brake lines to the rear of the car! They would need to equip with dual proportioning valves as well, remember, so those extremely rare situations in which the dual-diagonal design has merit and benefits ... don't justify the effort and cost to equip every vehicle!