1974 Imperial Brakes

[Corbin]

My '74 Crown Coupe is in need of a brake rehab. As discussed elsewhere on this forum and the other dark corners of the internet, several parts of this system are unique to the vehicle and possibly unobtainium: the rear rotors and master cylinder. Separating the master from the booster revealed at least part of the reason for my less than adequate stopping power: the master is leaking.

Queries to my local parts guys turned up nothing. (Well not entirely true-one place indicated a part in Michigan, but after waiting a few weeks for it to arrive the part in the box didn't match the part number on the box. Cue sad trumpets) Queries to my go to NOS vendors also turned up nothing. So I'm left with adapting something or sending the master out to be rebuilt.

In ordering parts I also ordered a master for a Chrysler Town and County (Cardone part number 10-1572). This part appears physically identical to the part removed from the Crown. Covers and gaskets are the same (contrary to the parts manual, which list different part numbers depending on Chrysler or Imperial), depth of the reservoirs are the same. The casting numbers look to be identical. I realize internally there should be a check valve for the rear on the non-Imperial master. Outside of that I can't see a reason not to remove the check valve and use the non-Imperial master. Any one want to chime in? Offer their knowledge? This car was in regular use until the late 90s, so I have no reason not to think the removed part is correct for the car (if not orginal). For reference the Cardone p/n for the Imperial specific master is 10-1575

 

[Corbin]

Some more comparison images....

 

[CBODY67]

Odds are, I would suspect, that Chrysler did not go to the expense of designing a specific-one-model use master cyl for a low-production situation (i.e., Imperial 4-whl disc brakes). Therefore, adapting something they already had in use, with a tweak, would be the most cost effective way to do things. In this case, removing the drum brake residual pressure valve in the rear circuit would be that tweak, to me. Perhaps others have other ideas OR have "been there" before?

Proceed at your own risk,
CBODY67

 

[Ross Wooldridge]

I would suspect the T&C master is fine, but to be sure, get yours rebuilt, and the booster too. Brake fluid is hard on boosters.

There was a guy selling the whole rear rotor setup here on the site some time ago.

Here's one hit: For Sale - 1974-75 Imperial Rear Brake Rotor

Here's another: For Sale - 74-78 imperial rear disc brake rotor and calipers

Here's yet a third: For Sale - 74-75 imperial rearend. Kenosha wi

And here's the system I was remembering - it's marked SOLD, but it's worth PMing the original poster to see if that's actually the case.

SOLD - REBUILT Factory "REAR DISC BRAKES" From A 1975 Chrysler Imperial 9 1/4" Axle.

 

[SGT FURY]

I still have the calipers and one rotor available for $100 for the three pieces.
I also have complete disc brake rearend still in car available. In 75 imperial

 

[Corbin]

Thanks for the input.
I think I have the calipers covered- upon noticing the failure of the master cylinder I ordered new calipers for all four corners. The fronts are in my possession, the rears are en route; One from each coast.
The surfaces of the rotors looked fine when I inspected them. The fronts measured out with lots of life.
I didn't measure the rears; the shrouding made it difficult. When I replace the calipers I can measure the rotors and go from there.

 

[Davea Lux]

That master cylinder should probably work, Car-don't has a very bad reputation for quality, so disassemble the unit and check it thoroughly. (Most of their stuff is rebuilt overseas by folks who do not read English) You would also want to check the bore size on both MC's to be sure that they are the same. Car-Don't is also famous for shipping the wrong parts in the right box.

Dave

 

[Corbin]

More information, mainly for posterity.
The check valve is contained in the distribution block/proportioning valve assembly. The bore diameter of the Chrysler cylinder is 1.032 nominal (avg.) The bore diameter for the Imperial is 1.064 nominal (avg). That works out to a nominal cross sectional surface area of .836 in^2 for the Chrysler cylinder and .887 in^2 for the Imperial cylinder. (I realize I'm not measuring the actual piston, and neglecting piston geometry) I'm also going to assume the stroke is the same between the Chrysler master and the Imperial master, meaning the change in volume (piston area x stroke) is the same between both versions.

That covers the difference in cylinders between the two vehicles. The FSM indicates the output pressure of both the master and the proportioning valve of Imperials (Y models) matches that of many other Mopar products in 1974. (Interestingly the remaining others are higher).

The T&C master has been installed in the car (along with new calipers in the front). The car seems to brake fine from freeway speeds; a little better than my Grand Wagoneer and a little worse than my modern 4 wheel disc brake equipped truck.
The bore of the failed master measured at 1.032-meaning the car had the 'incorrect' master installed when the correct parts should have been readily available.

I have a few more issues to wrap up (off idle stumble, possible ignition issue, a second round of fluid flushes) and she'll be ready for commuting duty. I'm sure other issues will present themselves as car is pressed back into service after its 20 year slumber.

 

[Corbin]

Images from FSM for reference.