"Invisible Crowbar" Beneath Brake Pedal of 1970 300

[fins2fuselage]

I have (and have had for 12 years) a problem with my unmodified "survivor" 1970 Chrysler 300 with 4-wheel power drum brakes: when the brake pedal is depressed to within about an inch or less of the floor, the pedal suddenly "bottoms" against something "invisible"! I realize that it must be hydraulic in nature, but it feels just as though there is a steel crowbar down there stopping the pedal's descent --- yes, the stop is that abrupt. Crack a bleeder and --- poof --- the crowbar is gone and the pedal hits the floor.

Unfortunately, the point at which the pedal stops is just about an eighth of an inch too little to allow the driver to feel comfortable while stopping on a steep hill: the brakes don't quite fully engage at that point, so lower gears and/or parking brake must be employed.

The problem began in the early spring of '04 after the car spent the winter under its car cover in the carport; I have inspected, replaced and bled everything (often twice) but, in all of these years, I have never stumbled upon the solution.

Has anyone else ever encountered this problem? And if so, how was it resolved?

Obviously, I'm new here . . . so "Hi". I have owned this car for 23 years --- half of its life.

Jeff in NC

 

[Samplingman]

Welcome to the forum, pictures of your car are mandatory! Sorry, not great with brakes myself, so hopefully an expert will chime in soon.

 

[fins2fuselage]

Thanks, Samplingman. Below is the only halfway-good photo I have of the car; it was taken at a car show last May. Antique Ivory (yellow) body, black vinyl top, gold fabric-and vinyl bench seat interior, slotted road wheels --- just as it came from the factory.

Jeff

 

[Samplingman]

That is a nice looking 4 door!

 

[fins2fuselage]

Thanks; it is a fairly nice car.

Jeff

 

[CanCritter]

what would l do...first thing l would do is bleed all the brakes and enshure you have no air and have NEW fluid...
would put car up on jackstands(rear) and check for brakes with tires on by simply grabbing and see if you can turn..check fronts as well....is just mater of elimination...checking brakes ect is just basic maintinece that needs to be done anyway so best to do it to enshure all is as should be and move on from there
next thing l would do is enshure theres vacuum and enshure check ball is where its susposed to be in the proportioning valve...may have been set off and not reset right...
if that's not the issue swap out the master cylinder
if at this point cant sort it out either querry folks here or run it off to a mech...
welcome to the site enjoy

 

[fins2fuselage]

Thanks, CanCrittter; all of that save for the distribution valve has been done (including the Mopar mechanic) at least once, some many times. As I said: this problem is almost 12 years old . . . and I am still fighting it.

Jeff

 

[Big_John]

Two things come to mind. First is an internal failure in a brake hose. Usually a bad brake hose won't let the brakes release, but I think it could go the other way too. The other is a bad master cylinder. Bad check valve, or piston/seals installed wrong etc.

My money is on the master cylinder.

 

[fins2fuselage]

John,

Thanks; it was rebuilt (by me) once, then replaced when that failed to cure the problem. The replacement didn't solve it either, of course.

Jeff

 

[1978 NYB]

Welcome.....nice car.

 

[fins2fuselage]

Thanks, NYB!

Jeff

 

[cantflip]

Hi, nice car.

Reading your symptoms and how/when it happened, I would bet a dollar you have one or more frozen wheel cylinders or a internally damaged brake hose. No offense to you or your mechanic, but it seems few people really know much about drum brakes other than basic servicing. I have worked around and redone the work of professionals for decades.

A stuck wheel cylinder may not be applying as much force or any to that wheel. A brake hose can get an internal tear and make a flap that acts like a one way valve blocking pressure from applying. A simple test would be to wedge a bar between the seat and brake pedal to apply some brake pressure and jack the front wheels one at a time and feel the turning resistance, then jack the back and feel the resistance in neutral. If the front or back are not applying, you have narrowed down where your troubles are.

You didnt mention anything to cause me to suspect anything else in the system. Wheels that dont bleed well or any work done prior to the concern would take my thoughts in other directions. If you dont feel comfortable doing this work, try a different mechanic... I would have never let this car drive away undiagnosed.

 

[cantflip]

Couple other things that dont match your above statement of what happened but would be good to check... if the wheels are all appying, the shoes could be on backwards... I have seem lots of "professionals" do that. Other wise I would consider new quality shoes, I have had cases where a customers discount brake linings did not stop well and a new set solved the problem.

Assuming you drive the car, I wouldnt think burnishing would be needed. To burnish the brakes you would make several low speed stops to scrub the friction surfaces together and "mate" them but it also builds some heat and outgases the linings releasing moisture and scrubs away flash rust. Always burnish new brakes and it would be a good practice for a car out of storage.

 

[fins2fuselage]

cantflip,

Thanks for all of the input; I'll try some of the diagnostic tricks you mentioned.

I am very comfortable working with brakes, so most of the work has indeed been performed by me; it was taken to the Mopar expert in 2005 or 2006 out of desperation. The problem developed in '04 after the car stood parked for about five winter months, but no brake work had been recently performed prior to that period of dormancy.

I know that the short lining goes in the front position, so I doubt that is the problem . . . but I can certainly check. Two different brands of shoes have been used in the course of chasing this problem, but that didn't make a bit of difference. This car had fantastic brakes from 1993 when I purchased it until spring 2004 --- and yes, the brakes were indeed serviced (by me) in the years in between. I even replaced all of the steel brake lines with stainless in 2000. Hoses were replaced in 1997 (and perhaps again in 2000 when the lines were changed), so deteriorated hoses really shouldn't have been a problem by 2004. I know that the wheel cylinders have been replaced, probably in 2000, and all brake hardware that could be replaced was (including adjuster cables); self-adjusters were disassembled, cleaned, bead-blasted and reassembled with anti-seize. You get the idea.

Jeff

 

[Big_John]

The hoses don't have to be old to have that issue and from everything you just said, process of elimination points to a hose or a wheel cylinder problem.

 

[65sporty]

I would still ck each wheel cylinder to make sure none of the pistons are froze up. How's the adjustment on the brakes?

 

[cantflip]

cantflip,

Thanks for all of the input; I'll try some of the diagnostic tricks you mentioned.

Jeff

It sounds like your doing ok, next time try SilGlide instead of anti seize... the silicone grease is far better with the heat that can generate in brakes. Regardless of age, brake parts cant be protected from rust... wheel cylinders are the lowest point and where any excessive moisture will collect. If you disassemble for inspection I alway push the pistons in by hand (dont let them pop apart) to make sure both pistons are moving. It would be rare to find one half frozen, but not impossible.

This wouldnt make sense with all you've said, but if you go in write down your casting numbers on the cylinders and master. Hydraulic pressure is created by the difference in size of the master cylinder pistons vs. the wheel cylinder piston. I dont know if there is another part the parts store could have mixed up that would be close enough to seem right... but if you dont find anything on inspection it could create your symptom too, but should have been apparent as soon as installed.

Good luck and let us know how this turns out please,
I'm Jeff too

 

[detmatt]

Welcome to the site from the Motor City! Very nice Chrysler.

 

[commando1]

when the brake pedal is depressed to within about an inch or less of the floor, the pedal suddenly "bottoms" against something "invisible

Nobody knows how to troubleshoot.



Booster.

 

[fins2fuselage]

I just remembered that the brakes do pull slightly to one side (to the left IIRC), but I can't recall a time when they didn't; maybe that's a clue.

It appears that I have some checking to do.

Jeff