1939 Plymouth Brake Pedal Goes to Floor

[Polsonator2]

I was driving a couple blocks from my house this morning and my brakes all of a sudden went out on me (yikes!!).  Pedal went to the floor and I was not able to build any pressure up.  By miracle, I had fixed my e-brake just a couple weeks ago so was able to slow down and stop, then limp it a couple blocks home using the e-brake. I checked the master cylinder and it was full so no leaks of fluid.  I could see the brake linkage working so am suspecting an internal master cylinder fail.  Does anyone have any input on what may be going on?  All brakes components were replaced a few years ago and car hasn't had any issues.  Sometimes over the last couple years the pedal could seem spongy but always built pressure back up after pumping the brakes but having never drove a car with a single master cylinder with no power brakes I figured maybe this was normal.  

Edited November 10, 2019 by Polsonator2

[desoto1939]

I have a 39 Desoto. Had a similar pbm.  The brakes worked fine for 30 years then one day sitting in the garage pumped the brakes to test in the winter and then pedal went to the floor.  Started checking the backing plates of each wheel to see if any leaking brake fluid and found none.

 

The next day got back in the car and was able to have a good pedal, very odd then the pedal went to the floor.  Pulled all four drum not leaking and wheel cylinders.

 

Knew it had to be the MC which as you know is under the floor board. Pulled that. 

 

Took it apart thinking that I might have a torn rubber seal.  No torn seals, no score or markings in the main  piston area.  So decided to do a complete rebuild and changed the piston from the aluminum to a brass piston. Put in a new check value.  Bench bleed the MC and installed the MC..

 

I then had good brakes with good travel as per specs.  The next day the old parts were on my work bench I looked at the check valve and noticed a very slight cut in the rubber part of the value.  This was the cause of my issue. The pedal when beaing pressed permitted the pedal to go to the floor and did not have a resistance.

 

So I hate to say this but pull the mc and rebuild.

Do you have any MC rebuild kits. Are you using DOT 3, dot 4 or Dot5 fluid.

 

Rich Hartung

desoto1939@aol.com 

[Polsonator2]

Thanks Desoto!  I bet my MC is probably having the same problem so i will definitely pull the MC first and rebuild it!  I have no rebuild kit.  There is a place not far from me (Hagens Auto in Puyallup WA) that specializes in rebuilding old brake components for vintage cars, especially cylinders.  Was thinking of getting a full rebuilt one from them and giving mine as a core.  Are the kits avail and are they easy to rebuild yourself?

 

I am using DOT4 but I think it may be mixed with DOT3.  Aren't these mix compatible?

[desoto1939]

you have the same MC as in my 39 Desoto. The rebuild kits are available and can be gotten from Andy Bernbaum.  The MC is very easy to rebuild so long as there is no scoring in the main area where the piston goes in and out.  You can rebuild one in less than an hour if this is your first time.  You can also get an new unit off of ebayMC complete for around 175 and that should have everything and just add brake fluid and bench bleed the MC prior to putting it in the car.  If you have more questions please feel free to contact me.

 

Also look on ebay there are MC rebuild kits, get one that has the piston. The 39 Mopar parts are not an off the slf item but there are more ply parts then Desoto parts.

 

Rich HArtung

Desoto1939@aol.com