Another brake question....Master Cylinder issue?

I think you have several things working against you. Heavier car, smaller diameter discs, larger master, manual brakes. I assume you have the correct manual brake linkage which gives more leverage vs the PB linkage.

A smaller master would help. Enough? Not sure. Too small and you may run out of fluid volume or pedal travel. 15/16" is the smallest I'd go. Dr Diff offers one in Aluminum. May be worth a call.

I may be way off base...but I dont think car weight is an issue. My car weighs no more or within a few lbs of the average BB e body. I have shucked quite a bit of weight off it. Just wondering...how does a similar system work in an e body?

Also, it has been my experience that an "under braked" vehicle usually locks the brakes easily? For instance, ( I know not an exact dollar to dollar comparison) my nissan pickup will lock the brakes way too easy if you put a trailer on it.

This car had 4 wheel manual drums from the factory. It didnt stop as "good" as the current setup, but it would lock all 4 if need be. The car weighs at least 300lbs less than those days.

As far as linkage, I have a pedal from a 71 RR manual brake car. The rod is, I believe, an adjustable rod from master power.

What are the measurements to verify that I have the correct pedal?

Could an incorrect pedal height, too low for instance, be helping cause the issue?

The car does stop pretty decent. On a short 1/8 mile track without much shut down area, I have to lock her down pretty good, but it does it fine. I just dont think that not being able to lock either front or rear is "correct"?

reading the original post, I do believe you are correct. you need a smaller bore mc.

here is why.
The amount of force you are applying does not equate to enough force at the calipers to lock up the brakes.
by using a smaller master, the math showed for me an increase of over 100 ft pounds of force applied at the calipers over the larger master that I was using.

since you don't have a booster, this is the only way to increase your brake force applied.

feets excel sheet shows that just by changing the master cylinder, no other differences, makes a huge amount of force difference.

I found that out too real world, that I could lock mine up after that.

I only had one choice because of the aftermarket brakes.
1 and 1/8 down to 1 inch.

that 1/8th was a huge improvement.

they didn't make a smaller one for my application. there were a few people that adapted some from other cars, but I figured that was a worst case thing, trying to find a 7/8ths.


and you can download it from http://3gduster.com/brakes.html

I mentioned the residual pressure valve to keep pedal play/throw to a minimum.

My daily Dart has a DrDiff MC (1-1/32") with large C-body calipers upfront and drums at the rear, but since I haven't installed a res.pressure valve yet, and therefore have some more pedal-movement in the system than I would like.
Besides that I also think I would like to try out a slightly smaller diameter MC as I think my brakesystem would benefit from a little more hydraulic pressure.

Interesting thread as I have the same problem now too. Have a 67 Satellite that I changed to front power disc from standard drum drum. Bought a 7 inch MBM from Nelcohotrods complete with MC (1 1/8 bore) and proportioning valve. Was out of warranty before I got to test the brakes and wow, worse than standard disc, literally could not stop this car in any reasonable distance.

Tried to get help for Nelcohotrods and they would not give any suggestions or assistance at all. Was to be made in America parts but I find out today the proportioning valve is China made.

Spoke to National Moparts today and he says those proportioning valves are mostly bad right from the start. He has replaced something like 40 brand new ones that never worked. He gave me a replacement to try and said I need to get down to a 15/16 bore too.

This supports what most of you are saying on here. Smaller bore master and check the PV.

you said you tried two different 15/16 units. I was told there is no such thing as one of these. I assume they are a master from a standard brake system and not from power. What did the 15/16 MC fit.