1939 Plymouth Deluxe brakes light comes and brake after 15 minutes of driving

[38tech]

I'm assuming during the first 15 or so minutes you've had to use your brakes and then the problem ocurrs.
If that's the cas, I suggest you follow David Pollock's & Tod Fitch's advice. The problem is in the master cylinder relief port. The tiny vent port is partially plugged or plugged solid, not allowing the pressure to relief itself after the brakes have been applied.

[jkopy]

in reference to access to the m. cylinder, when i rebuilt the brakes on my P6, i did it from underneath the car.? what a pain (unless you have a lift, which i don't).? next time i'll be taking the floorboard out!?

good luck

[plymouth39]

i HAD A PLYMOUTH CLUB MEMBER COME OVER AND HE ADJUSTED THE PUSH ROD IN BACK OF THE MASTER CYLINDER.  THAT SOLVED THE PROBLEM FOR ABOUT A MONTH AND A HALF.  THE BRAKES ARE LOCKING UP ONCE AGAIN.  COULD IT BE SOMETHING ELSE BESIDES THE MASTER CYLINDER BECAUSE WHEN I LET SOME OF  THE BRAKE FLUID OUT FROM THE THE FRONT RIGHT WHEEL, THE PEDAL IS STILL PRETTY FIRM.  ALSO, THE BACK WHEELS ARE THE ONES THAT LOCK UP  WHEN I START TO DRIVE THE CAR.

[David Pollock]

Tod Fitch has your answer.  Read his message about the relief port.

To prove the problem is in the master cylinder, drive the car, or pump the pedal until the brakes and the brake lights stay on.  Then open one of the bleeders on any wheel cylinder. Relieve the pressure and close the bleeder. All should be normal for a short time and then the problem will return.

  A blocked relief port makes it difficult to purge the top end of the system of air. This trapped air allows the brakes to release as normal for days, weeks or months but when it finally escapes, the brakes will stay on and the pedal will feel harder and harder.  Usually when it is least convenient.   dp

[Bob Kinker]

I had this problem on my 40 Ford. Turns out the wheel cylenders were not fully retracting fully.  I guess crud had built up from extended storage. I replaced all wheel cylenders, master cylender, and hoses. Also flushed the system and refilled with semi-synthetic brake fluid. Been working great ever since!
Bob

[Jim Yergin]

You should not have to remove the floor. You should be able to reach it from underneath or from in the engine compartment.

[plymouth39]

Due you have to remove the floor board to gain access to the master cylinder to make the adjustemtent or can you gain access underneath the car?

[Wm Steed]

I believe I still have the original peddle assy, brake rod & m. cylinder from my 39 Plym. I will try to find it today so I can check the rod as to if it is adjustable or not....Bill

[Jim Yergin]

The rod on my '41 is adjustable. I have also experienced the brake lock up when the rod was not adjusted correctly.
Jim Yergin

[TodFitch]

On the 1933 PD the rod from the brake pedal to the master cylinder is adjustable. Which is how I learned about blocking off the relief port when I was trying to get a higher pedal height.? ;D

I have mine adjusted to where it just touches the piston. No real slop but not pushing it in at all.

From the sound of the posts that followed mine, it seems that the rods on later models might not be adjustable. Is that true?

[Ken Bartz]

Bill,
Could you tell us the length of the master cylinder rod?  I'm having the same problem with my 40 Plymouth.  I'm had the master cylinder out four times.  All of the other wheel cylinder parts are new.

[Wm Steed]

I guess my posting will just re-inforce what Tod has basicly said, in any case here goes...........

I have personally encountered this problem twiice in years gone by. Once it was a faulty M. cylinder which would not allow the pressure to bleed back into the cylinder. As I drove the vehicle the pressure would slowly build up until the car would not move, solution...... carry a 3/8" end wrench so I could open a wheel cylinder and release the pressure so the car would move. Replacement of the m. cylinder solved the problem.

The second experience was due to the wrong rod from the peddle assy to the m. cylinder. The brakes worked until I did a complete brake job, shortly there after  the brakes would slowly lock up. After replacing the m. cylinder twice I checked the rod, I compared it with a known stock rod and found it to be longer by about a 1/4", replacement of the rod solved the problem.

There should be some "FREE TRAVEL" between the rod end and the piston in the m. cylinder, not much but enough to allow for heat expansion, etc....Bill

[TodFitch]

Is it one wheel that is locking up or all of them?

I have heard that brake hoses can sometimes collapse internally so that the pressure from the master cylinder gets into the wheel but can't return.

But, more likely in your case, the rod from the brake pedal arm to the master cylinder is probably incorrectly adjusted or there is something blocking the relief hole. There are two holes in the bottom of the master cylinder reservoir. The one in question is a pin sized hole on the side away from the pedal (toward the outlet side of the master cylinder). Its purpose is to allow the extra fluid in the system to return to the reservoir. If it is blocked then when the brake fluid expands from heat in the system pressure will build up. That pressure will apply the brakes (creating more heat) and close the contacts in the brake light switch.

Verify that the pinhole is clear of debris and that when the brake pedal is released that the master cylinder piston cup returns past the pinhole.

[plymouth39]

I have 1939 plymouth that after driving for about 15 minutes, the brake light comes on and the brakes stick.  I was told to let brake fluid out of one of the wheel brakes lines and it would stop due to pressure build up.  I did that but within fifteen minutes the light and brakes due the same thing.  Any sugestions as to what it causing this and what do I need to do to correct it.