RE: [Chrysler300] Problems with AAJ power disc brakes on a C300
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

RE: [Chrysler300] Problems with AAJ power disc brakes on a C300

The MC and boost are newer. An AAJ recommended dual diaphragm 1980’s Chevy unit. It fits well between the dipstick, power steering pump and air filter. (for those cc’d see photo)


The general consensus is that the just installed boost unit is defective and/or the 300’s cam offers inadequate vacuum to operate correctly.


More later.  …Jack




From: Rich Barber [mailto:c300@xxxxxxx] 
Sent: Tuesday, March 7, 2017 1:11 AM
To: 'George McKovich'; 'Matt Allyn'
Cc: 'Jack Boyle'; Chrysler300@xxxxxxxxxxxxxxx
Subject: RE: [Chrysler300] Problems with AAJ power disc brakes on a C300


The big rubber diaphragm inside the one-year-only Kelsey-Hayes power brake boosters on 1955 MoPars is subject to deterioration and rupture.  On a good day, these boosters do not work so well.  There is a reason for that 9-inch wide brake pedal.  Of course, C300 boosters are significantly different from 300C boosters but are sensitive to end clearance on the rod.  Clevis to firewall specification is in 1/64ths.  Fluid can also accumulate in the booster unless the vacuum connection is facing downward.  Disc brakes may require more fluid displacement from the MC.  The ’55 MC’s worked differently with a metal piston displacing fluid from a pressurized cylinder to the system.  Never caught on.  See:     Page 96.


I believe proportioning valves are essentially variable orifices and may not work so well if set really tight.  “normal” master cylinders have a device referred to as a “check valve’ in the outlet to hold a little pressure on the brake shoes with no foot pressure on the pedal.  This is to minimize moist are migrating back into the wheel cylinders.  I don’t know if such a device is fitted to the K-H MC but it could be part of the problem.


Many may be interested in a successful front (?) disc brake conversion for ’55 C-300’s.  Please keep us posted as to the details necessary to achieve proper operation.


Rich barber

Brentwood, CA



From: Chrysler300@xxxxxxxxxxxxxxx [mailto:Chrysler300@xxxxxxxxxxxxxxx] On Behalf Of George McKovich george@xxxxxxxxxxxx [Chrysler300]
Sent: Monday, March 06, 2017 7:11 PM
To: Matt Allyn <allynentertainment@xxxxxxxxx>
Cc: Jack Boyle <jackcboyle@xxxxxxxxx>; Chrysler300@xxxxxxxxxxxxxxx
Subject: Re: [Chrysler300] Problems with AAJ power disc brakes on a C300




I go with the problem being with booster. I imagine you pulled the booster off the firewall during the installation, as I did on my 300C. I do not think any boost is getting to your peddle. Mine acted the same way... I could go 30mph and stand on the peddle with both feet and my full 180 pounds of weight and I would coast to a stop but barely. What I discovered on my C, was as follows:


If you remove the booster, you would likely find that the little "tang" at the very end of the peddle has been bent or twisted such that it does not work against the little white button on the back of the booster, where it sets between the two blades, one on each side of the top of the pedal arm. This can happen when installing the booster blades or prongs through the firewall... it is very easy to have happen. Remove the booster and examine the upper end of the pedal arm, making certain it has a slight curve and rocks back and forth and is straight in the slot it fits into.


When you get the booster ready to mount, have a friend get below your dashboard with a flashlight and watch that those blade or prongs go on both sides of the pedal arm while you place some bolts attaching the booster to the firewall. Before you get it completely tightened up, check from under the dashboard that the tang at the upper part of the arm touches the button on the back of the boaster when you take your foot off the pedal. When it touches, it releases the vacuum. If the button isn't pushed and vacuum isn't released it can't gain access to more vacuum. 


I can send some pictures later tonight showing what I am talking about. If this isn't the solution, get back to me and I have some other ideas.


George in snowy and windy Idaho!

On Mar 6, 2017, at 6:59 PM, Matt Allyn allynentertainment@xxxxxxxxx [Chrysler300] <Chrysler300-noreply@xxxxxxxxxxxxxxx> wrote:


That pedal feel describes what I experienced when my power booster unit went bad on my '62. Make sure yours is working?

Matt Allyn

On Monday, March 6, 2017, 5:20 PM, 'Jack Boyle' jackcboyle@xxxxxxxxx [Chrysler300] <Chrysler300-noreply@xxxxxxxxxxxxxxx> wrote:


I just finished installing a disc set up from AAJ on my C300. There were
some bumps along the way but it’s finally complete. Now I am having poor
braking performance. The symptoms are primarily weak braking with both feet
on the pedal – and no lock up, not even a chirp. It’s like I don’t even
have power brakes. The pedal is as hard as a rock and never even gets close
to the floor.

Roger at AAJ and my local old Mopar expert have offered some suggestions but
before I act on those I wanted to poll the group.

Here is what I have done to date:

1. New DOT 5.1 all around (glycol type with low moisture absorption)

2. Bench bled the MC per instructions

3. Found one line leak and fixed it (defective NEW! fitting)

4. Adjusted the rear drums as close as possible

5. Checked the booster vacuum line for obstructions

6. New stiff factory dia. Vacuum Hose with no leaks

7. Vacuum tests at 14 inches, 15 max. (damn cam)

8. Bled all wheel cyl. 3 times starting with the RR

9. Tried all ranges of the proportioning valve setting, including closed
off completely (best braking is with valve open ¾ turn)

Still a rock hard pedal and VERY marginal braking performance.

Any tips, questions, fixes, etc. welcome.

Thanks …Jack

Jack Boyle

(913) 544 4650 <tel:(913)%20544%204650> 

Enjoying the same C-300 since 1967

IMG_0623 small

[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]

Posted by: "Jack Boyle" <jackcboyle@xxxxxxxxx>

To send a message to this group, send an email to:

To unsubscribe from this group, send an email to bob@xxxxxxxxxxxxx or
go to

For list server instructions, go to

For archives go to

Yahoo Groups Links

<*> To visit your group on the web, go to:

<*> Your email settings:
    Individual Email | Traditional

<*> To change settings online go to:
    (Yahoo! ID required)

<*> To change settings via email:

<*> To unsubscribe from this group, send an email to:

<*> Your use of Yahoo Groups is subject to:

Home Back to the Home of the Forward Look Network Archive Sitemap