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66 Coronet power disc brake conversion mismatched parts?


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1:18 AM
Sep 13, 2023
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i'm a long-time non-registered lurker on the "for X only"-forums, mainly to gather information for my former W200 and W250 trucks.

Now i own a 1966 Coronet four door sedan, 318, 727, 8 3/4 that's a rolling renovation and modification in progress.

I would like to ask some questions on the braking system, especially concerning the pedal/booster/master setup in a disc brake conversion.
I have read a few threads discussing the topic but could not find a solution for my particular problems.

My Coronet was a manual drum brake car.
When i got the car the master was leaking externally (into the car) and while the braking power was ok, i was not too content with manual drums in city traffic or general Autobahn-madness (i live in Germany).

Fast forward a few weeks and a complete MBM power brake disc conversion set found its way into the car.

This is the kit (i bought it through a local dealer):

It's got the 11" rotors, sliding type calipers, large dual diaphragm 8" booster, 1.125" master cylinder, firewall plate and linkage:


I also installed the dealer provided distribution block and metering valve shown:


I have had lots of problems with the master cylinder and booster in this particular kit, making me question my choice to convert and the overall setup despite its originality.

First of all, the master leaked internally because of massive scratches and grooves the front piston (which is for the rear drums) seals ride.
This resulted in lots of pedal travel and a lack of rear brakes.

I changed out the master for another new one, same 1.125" diameter bore, bench bled till the cows came home and i got a slightly better pedal feel to it but here we get to the second massive problem.

With this large booster there is almost no control over the pressure applied to the brakes, touch the pedal and move it a hair too far and the brakes lock up violently!
It feels as if you crashed into a solid wall, it's that violent.

I assumed the booster might be defective as well, i experienced a similar thing with a rebuilt booster for one of my trucks.
As far as i can read a cracked internal hub will make the booster uncontrollable and it will be either totally off or full power on the brakes.

I did some checks, mainly concerning engine vacuum, bled the brakes some more (didn't really find anything).
My engine makes a solid 17in hg of vacuum at idle, valve guides are worn a little but nothing that concerns me much.

So i spent a bit more cash and got a rebuilt 1970 Coronet single diaphragm booster from Rockauto (CARDONE rebuilt), this unit here:


To my knowledge this is one of the factory installed Midland Ross type boosters.
It appears in the FSM i have for the car.

This unit works, can be modulated and seems to be correct for the application.
There's a "but" coming though and this is my main concern:

The pedal feel with the linkage shown above, connected to this Midland Ross booster driving a new 1.125" master cylinder is too firm.
The braking power is good but it needs about as much foot pressure as the manual drums did which doesn't seem right to me.
Wayne at the RamManInc speaks about the pedal having about 1 - 1 1/2" of travel, which mine does but it's way too hard comparing to my 76 W200 and 85 W250 truck.

My questions are:
- What's wrong with the dual diaphragm booster? Is it defective (i am thinking of a way to test that thing)?
- Is my current setup with the Midland Ross booster, linkage, metering block and 1.125" master cylinder mismatched?
- What's the pedal feel supposed to be?

Sorry for the long winded description, i tried to provide as much detail and info as i can.
I'm a bit stumped with this conversion, i am thinking it should not be that hard and somethings just wrong or defective but i can't find it.

Any help is greatly appreciated, thanks in advance.

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