I've seen two 41Chevys with disc brake conversions done with the original master cylinders. The cars drive fine which amazes me since all the mopars I've done have never worked with the dual passage drum master cylinder.

Do most of the after market disc brake conversion smaller brake fluid capacity? Or are these guys just getting lucky.

Lucky, I bet during hard braking they'll swap ends.

-Daty

once the system is bled and topped off there is no need for additional reservoir capacity, until the pads wear. I doubt the drums were self adjusting, which would not require added capacity as it mechanically compensates for wear. But with non-self adjusting drums the brakes get worse at stopping and if you pay attention you know to adjust them.

Lucky? Nope, they either haven't run the pads down enough to run out of capacity or they are staying on top of the brake fluid level. Is it smart? Nope, lazy/cheap/ignorant take your pick.

A lot of the old cars had very large wheel cylinders. Its possible the master cylinders had enough capacity to accommodate the disc brake calipers fluid requirement until the pads wear significantly, brake lining wore off the brake shoes back in the day as well, resulting in lower master cylinder fluid levels even in the old drum brakes. If a person keeps up with the fluid level, I don't see an issue. Don't you keep the fluid level up on your modern disc brake cars? I find it hard to believe any real car guy wouldn't check and refill the master cylinder between brake jobs at least a few times.

It really is a question of capacity. The master cylinder has to have enough capacity to accommodate the fluid requirements of the brake system under the braking load, a little extra reserve make us all feel better. This is much easier when everything works as it should, but when things go a muck, systems on the fringe of function get scary. Sometimes mixing the design function of parts of a system (like swapping disc brake calipers where drum brake wheel cylinders used to be) puts things on the fringes of function. I suspect the major question here is, how close to the minimum reserve brake fluid capacity are you willing to live by?

I suspect the new brake systems are closer to the minimum then the old stuff was (as originally designed), in the old days most stuff was overbuilt. These days, the bean counters are in charge, now things are designed to get by with the least possible over build.

A comparison of the fluid capacity between the two wheel cylinders and the two calipers, and the capacity of the master cylinder might revel some interesting results. If the rear brakes have been updated, the capacity differences of the rear wheel cylinders also needs to be accounted for. Without real, factual, numbers, its all just speculation. Gene

gene has made some very good points. but what i see as a somewhat critical issue is the fact that using a duel circuit master will leave a person with at least some braking reserve if a problem arises, whereas if a single cuircit master develops an issue, you have zero brakes. remember this : "speed never kills - it's the sudden stop that does you in every time......"

it takes more fluid to activate the wheel cylinders than the calipers---check out how small the reservoirs are on masters used on circle track cars (speedway catalog)these are the same as used on nascar units in the 70-80,s...yeah and if i wanted to look more period correct i could use the stock pot on my 64 to activate my 4 wheel disc setup....bob

after i replied i thought about my 2000 caravan master. gene is correct. if i'm not mistaken, the caravan is "cross braked". in that the right front and left rear circuits are connected and the left front and right rear are connected. the resivoir is a [kinda] divided detachable plastic piece in that using a single fill tube [also detachable] to fill the resivoir, the divider wall is not very tall, so when you lose fluid for whatever reason, it doesn't take much loss to lose all braking action because it is not a truely divided system. perhaps the abs accumulators are now considered the safety net ? i am considering using 2 small remote resivoirs to plumb a dakota style aluminium master to create a true "duel pot" master for my humpback.

self adjusting brakes still use more fluid than a caliper---and cars dont swap ends unless there is more braking in the rear than the front--so if you had only a brake on the right front wheel the car would follow the brake on the right front------if you want to swap ends on your car you can go 60 mph down the road and slam on the rear emergency brake----if you dont move the steering wheel in correction attempt the car will turn ass backwards following the friction of the braking rear end....

the same follow the friction rule applies to the old interstate system---concrete has the highest friction ratio followed by asphalt then dirt----if you have a normal functioning brake system and drift over the line from the main thoroughfare (concrete) to the asphalt parking lane--left wheels concrete--right wheels asphalt and slam on the brakes---your car will follow the friction and all wheels will be back on the concrete----the same is true with left wheels asphalt and the right wheels on dirt---under full braking the car will climb back up onto the asphalt----that is why the engineers built the interstate system like this....

in the modern nascar days---when all wheel discs applied and before ceramic brakes and brake line pressure adjusters/restrictors----the guys would set up their cars by using a wood chisel to chip off pad material on the right front pads and chip off more on the rear pads---in this way the unaltered left front pad would drag the car around the corner everytime setting up the fastest corner speeds---on the really short tracks some would set an extra brake pedal to activate only the left front wheel to set up the turning aspect while the right foot kept you at full throttle...

The actual thread started because someone had a 40s GM car that had disc brakes attached to the original single pot master cylinder. I did indeed talk about the amount of fluid required to operate wheel cylinders compared to the amount of fluid required to operate disc brake calipers. The reality is, the master cylinder only needs enough reserve fluid capacity over the amount required to operate the brake system, to account for pad and shoe wear, and a bit of reserve. The stated concern was if the single pot master contained the amount of fluid required to stop the vehicle with disc brakes instead of the original drum. A comparison between the amount of fluid required to operate a pair of wheel cylinders and the amount of fluid required to operate a pair of disc brake calipers is relevant to this topic.

It is also very likely, the 40s GM brakes were not self adjusting. Seems i remember needing to adjust the brake shoes on nearly everything up to about the mid 60s. I don't believe the 11" drum brakes on my 1st 69 Road Runner had self adjusting brakes.

As for how often I check the fluid level in the master on my daily driver, the answer is, it gets checked at every oil change and before any longer then normal driving excursion, and gets topped off as needed.

Do you really not look at the fluid levels on your daily driver?
Gene

most probably the 40 brakes had the hach? system---this used two eccentric bolts to adjust the brake shoes out to contact the drum--they had to be checked regularly---

I have a '50 Olds with a drum/disc combo plumbed to the original under-floor single MC; I drilled the MC cap and threaded it for a nipple and hose connected to a remote reservoir. Many others with similar setups do the same.

john, what resivoir did you use ?

I used a polished billet aluminum reservoir I got off of e-bay....mounted on inner fender panel.

thank you !