1939 Brake specification

[rick55]

Thanks.  I did see that procedure.  I was looking for an actual number as a starting point.  The pedal is firm but seems a little low. It stops even with the gas pedal. I have very little free play at the moment.  I was concerned that increasing free play would cause a lower brake pedal. After replacing wheel cylinders and brake shoes the car stopped poorly.  I disassembled the brakes after the road test and noticed they were not making full contact with the drums.  I arced them using adhesive sandpaper inside the drums.  The shoes now make full contact with the drums.  I spent a lot of time adjusting the brakes to make them concentric with the proper heel and toe clearances.   I'll adjust the free play and see if there is any improvement. It will be awhile before I can drive it due to the weather. I'm new to these style brakes so I have a little learning to do.  Thanks again.

[TodFitch]

I don't know which version of the factory service manual you have but in my 1936-42 Plymouth Factory Service manual the brakes section starts off with:

1. Brake Pedal Free Play
1936-42
(1) Free play ("A," fig. 1) in the brake pedal should be from 1/4 to 3/8 inch. This free play may be readily felt by hand and is the movement of the pedal before the push rod touches the master cylinder piston. If necessary, this free play may be altered by changing the length of the piston push rod.

CAUTION
Piston cup (16, fig 13) must clear port ("B," fig. 13) when the pedal is in released position.

Basically, you adjust the length of that rod so that the brake pedal moves about 1/4 inch before the rod touches the back of the piston in the master cylinder. The rod must be clear of the piston when the pedal is released so that the piston cup has cleared the relief port ("B" in your figure) when your foot is off the brake. Failure to have that relief port working properly will cause your brakes to lock up.

[rick55]

I've been unable to locate the master cylinder push rod length specification for my '39 Plymouth. The measurement in question is shown "D" in the service manual.  I'm trying to sort out brake issues on my car and want to get everything adjusted to factory specs.  I don't know if the length has been changed in the last eighty years. Any help is appreciated.  You cannot view this attachment.