1948 Chrysler New Yorker brakes

[Akstraw]

I replaced the wheel cylinders, hoses and shoes on my 48 Chrysler.  I did not replace the master cylinder, as I had evidence that the prior owner had done so.  I flushed and bled the system with a pressure system, applying 15-20 lbs pressure at the master cylinder reservoir.  

On the 7-mile test drive, the pedal was firm, and became firmer.  At the end of the drive, at least one wheel (RF) was locked up and smoking.  Pedal was very firm with almost no travel, and the pressure-activated brake lights were on constantly after the car was parked and shut down.

Where to look for the issue, and what could it be?  Seems like maybe something wrong with the master cylinder.  Any thoughts/ideas?

 

[F&J]

10 minutes ago, Akstraw said:

Seems like maybe something wrong with the master cylinder.

Symptoms point to the probability that the brake push rod was adjusted too long, or was not adjusted to fit the new Master Cyl.   If the pushrod is too long and is slightly holding the piston in, then the piston covers the "fluid return hole" and then the brakes are not releasing all the way.

 

You need free play between the rod and the piston. You can feel freeplay if you use your finger on the pedal and go slowly a few times.  just keep adjusting the rod shorter until you feel free play with your finger.

Edited August 5, 2022 by F&J (see edit history)

[Oldtech]

If the pedal is holding the piston down a little it could do that. Be sure there is a slight "free play" between the pedal and master cylinder.  There is a very small "pinhole" in the bottom of the reservoir that must be open to allow the pressure to bleed back. If the piston is covering it or it is plugged it don't work!

[Ltc4748]

Check that the pedals pushrod is not holding the plunger down in the master first, also pull the boot back and verify the plunger is coming the hole way out and touching the snap ring. If these 2 items are correct check all the wheels for freedom of movement and then take it for another drive to see if the brakes apply again. Take a wrench along that you can use to take a bleeder or line loose, at any point that is easy to get to. If the brakes start to get hot again, crack the line or bleeder loose and watch to see if there is a little pressure forcing the fluid out. If you do this and fluid comes out then slows I bet the wheels no longer drag and build heat. The cause is the residual port in the master is blocked. If you look inside that master cylinder there are 2 ports that allow fluid to go into the bore. One is about 3/16" in diameter, the second port is only about .040" the smaller port is what allows the brake fluid to return to the reservoir when the brakes are applied and the fluid expands from warming. I have seen vehicles experience this and the further the owners ran them the more the brakes applied. By releasing the fluid pressure the brakes will quit dragging and the heat will no longer keep building up. Since it is a single brake system (unless the previous owner upgraded to a dual master cylinder) taking any line loose will drop the pressure in the entire brake system. To fix you can take a bread poke wire or the smallest torch tip cleaner and push it through the small hole to open the port. The trick is getting to it if it is not below the cap. I have seen some that you had to take the master apart to clean it out.

Keep on drivin that 48!

[Ltc4748]

When the fluid is cold the wheels will be free, as you drive the car the fluid expands from heat and slowly build pressure if the port is blocked by incorrect adjustment or rust/dirt in the port, crack a line the pressure drops and the wheels will rotate freely again. 

[Akstraw]

Shortening the push rod seems to have done the trick.  Thanks for your replies.  You have saved me a day of research and headscratching!