Braek Upgrade-Need Late-Model Mast. Cylinder?

[richemj]

I'm in the process up upgrading my '71 stock brakes with S alloys up front and NOS M fronts in the rear. Is the stock early-model master cylinder big enough for the job? I hear guys going Carrera and 930 brake on early models need the larger master cylinder, just wondering about this setup. System was fine with alloys up front and M rears in back, should using the M fronts in back make that much more volume to render the cylinder not functional?

Thanks much!

[ttweed]

The stock, dual-circuit, 19mm master cylinder from '68 on will work fine for you. 930 4-piston calipers need the 23mm master cylinder, but Carrera and smaller 2-piston calipers have plenty of pedal with the stock MC. Not a good idea to use the M front calipers in the rear, though, if you are just putting S calipers on the front, from a bias standpoint, unless you are installing a proportioning valve for the rear circuit.

TT

[richemj]

Thanks for your comments.

I had some clarity after all the brake fluid passed through my system. Essentially M fronts in the rear with alloy S up front would give close to a 1:1 biase, not real smart. Threw the M rears back on last night... brakes hard and smooth now... lesson... don't mess with factory S specs!

[Neunelfer]

Thanks for your comments.

I had some clarity after all the brake fluid passed through my system. Essentially M fronts in the rear with alloy S up front would give close to a 1:1 biase, not real smart. Threw the M rears back on last night... brakes hard and smooth now... lesson... don't mess with factory S specs!

Good idear...

You'll be fine with the 19mm as you haven't changed anything. M and S calipers "could" literally share the same 48mm pistons. You've gained:

Larger/Deeper Pads
Lighter Calipers

[richemj]

After installing the M fronts in back (with S alloys up front), I was wracking my brain thinking I was chasing air in the lines for the pedal would go to the floor even after a number of reservoir refills - using stock 19mm master cylinder. Switching back to M rears in back, pedal almost immediately built pressure, thus, 19mm master cylinder wasn't supplying the needed volume to move all the bigger calipers (48mms in front and back with this setup). I hear going Carrera fronts and SC rears (a good track setup) allows one to retain the stock 19mm MC. May try that way down the road. I like the Alloys too much though.

[ttweed]

Switching back to M rears in back, pedal almost immediately built pressure, thus, 19mm master cylinder wasn't supplying the needed volume to move all the bigger calipers (48mms in front and back with this setup).

Mike-

Something else was going on with your bleeding process, I think, it wasn't that the 48mm pistons in the rear were too big for the MC to fill and build pressure. It might have taken longer, for sure, but it would have eventually pumped enough fluid to do the job. Are you sure the MC didn't lose its prime or you weren't allowing it to return completely on the up stroke so that the inlet port could refill the cylinder or something?

I used a stock 19mm MC for awhile with a 930 4-piston caliper conversion on an early car and it still bled them and worked OK, and that is with twice the number of pistons, although they are smaller at only 38mmF and 30mmR, it is more total volume than you had to fill. The only problem for me was that the MC produced too much pressure and too little volume, rendering a poor pedal "feel". The pedal was quite long, and very sensitive--like having power brakes with a lot of slop at the top before they kicked in--very difficult to modulate and easy to lock up in a panic stop. If the brakes would have started fading (which they never did, even in hard use), there would have been little reserve capacity in the pedal stroke as the fluid boiled.

I eventually changed to the 23mm ATE Mercedes truck master cylinder, which gave a very high, very firm (as in almost rock-hard) pedal, which was easier to modulate, but required a LOT more leg effort to produce the same braking force, due to the lower pressure of the bigger MC. I did use the 19mm for almost a year, though, on street and track, and it did work, though not optimally. IIRC, Jack Olsen went thru this same process when he first changed to 930 brakes, leaving the stock MC at first but eventually upgrading it for less compliance and better feel.

TT

[Mr9146]

I run the 19 master in my car with turbo brakes at all four corners. I've never been left wanting for more feel or stopping power ... I've grown VERY accustomed to threshold braking and the setup seems to be working fine for me.

The exact same setup in a full-blown racecar never gave a lick of trouble or left me wanting for more brake, either.

Drive 'em in deep...

[ttweed]

... I've grown VERY accustomed to threshold braking and the setup seems to be working fine for me.

Exactly what I found, Marco. As long as you adjust your driving style to the pedal feel, it works fine. I was worried at first about leaving the stock MC in when I upgraded, as the common wisdom was that you must have the 23mm one, but I had heard Jack mention that he used the 19mm for awhile, so I tried it. As long as you are smooth on the brakes and don't stab them in a panic, you can adjust to the longer throw and higher pressure.

It is really a matter of driver preference, and in some ways, having the smaller cylinder was better--it gave that "power brake" feel that required much less pedal effort, and the extra throw in the pedal made heel-and-toe easier, as it was closer to the gas pedal level. The extra compliance in the system wasn't really a problem as I had good cooling to the front brakes and on a light car, the 930 brakes are probably overkill. I waited until I was putting in an alloy front crossmember to upgrade to the 23mm MC, as it makes the fit easier, and used the 19mm for quite awhile with no problems.

The only advantage to the higher pedal effort required with the 23mm cylinder was that it was harder to lock the brakes up accidentally when you miss your braking point and stomp on the pedal too exuberantly by mistake (or in a panic stop, where a brain-fart might let smoothness fly out the window in favor of self-preservation!), and that the broader range of pedal effort made modulation in trail braking easier. In a heavier car, that might test the thermal capacity of those big rotors a little more, the extra pedal height and throw might inspire more confidence in long, hard sessions on a hot day or an enduro situation on a heavy-braking track, if fade began to show its ugly head.

TT

[john rice]

If you run the big MC without a booster you had BETTER have strong leg muscles. I went through this a couple of times years ago. My brother properly resolved the potential issues by doing a complicated PIA 23 mm MC With a proper booster. LOTS of work but best of both worlds.

Conversely, I still remember driving a friends 914 with a ripping Corvette engine and he was running the stock 19mm with 930 brakes. I ...wondered. .. I drove the beast and it felt great! From what I understand the only problem is that you could run out of pedal if you are using them hard enough. I cannot remember anyone who had this happen though.

After driving my brothers hot rod I would say that the 23mm WITH the booster is the absolute best.
JR

[911pcars]

I believe this 22.2mm VWCorrado MC will also work. It's shorter than the MB version and provides more clearance with the front crossmember. Piston area is a little smaller than the 23mm MB version too.

Sherwood