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: http://www.jholst.net/55-service-manual/brakes.pdf 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.
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!
Posted by: "Rich Barber" <c300@xxxxxxx>
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