48 Master Cylinder

[cadillac60]

I need a measurement for the operation of a new installed master cylinder. If you have a 48 Cadillac the measurement that would help is from the centre of the clevis pin in the pedal, to the serrated area on the rod that activates the master cylinder. This would take the guessing out of the trial and error method--it would certainly be appreciated.

Bruce W

[Bill Ingler #7799]

Hi Bruce: Did you get the master cylinder adjusted or do you still need a measurement?   Bill

[cadillac60]

We would appreciate the measurement,,

Thank you

Bruce W

[Bill Ingler #7799]

Hi Bruce: The measurement of 3 & 5/16 inch was taken from my 47 convert. Since the 48 shows a different frame number, use the measurement as a guide only as placement of the brake pedal and master cylinder could well be different on the 48 making my measurement invalid. Here is what I would do in your case to set up the master cylinder connection. Please familiarize yourself with the master cylinder schematic below. The correct placement of the piston ( 0) determines if the pressure relief hole in the bottom of the master cylinder is open before, after applying the brakes and after releasing the brakes. Picture 2, shows the master cylinder assembled with the boot( T ) in place as well as the clevis ( V ) and nut ( U ) screwed on to ( P ) the link. Without being connected to the brake pedal, the link is not putting any pressure on the piston keeping the pressure relief hole open.

Now look at picture 3, hoping I can get 3 pictures on this posting. This picture is looking down inside a master cylinder. The small hole to the left is the pressure relief hole, fill hole, bypass hole- this hole has a lot of names. You can see that the small hole is open. Someplace between that small hole and the large hole is ( N ) the rubber cup on the front end of the piston. You can see part of the piston in the big hole. Now when you step on the brake pedal it pushes the link which in turn pushes the piston forward. As the piston moves forward it will force brake fluid up through the small pressure relief hole until the cup (N)  passes the small hole.  With no place left for the brake fluid to go, the continuing pressure from the brake pedal will force the wheel cylinders to expand the brake shoes and we have brakes. When you let up on the brake pedal, the large return spring on the brake pedal as well as spring inside the master cylinder will return the piston to the original starting point and allow any pressure to push the brake fluid back up through the pressure relief hole. Now there is no pressure on any brakes coming from the master cylinder. I will follow this posting with another showing you how I would set up your master cylinder linkage.  Bill       

[Bill Ingler #7799]

Bruce: Assemble your master cylinder like shown in picture 2 above. Now look at the picture below which shows the brake pedal shank with the rubber grommet attached. You will also have a return spring attached to the brake pedal. Allow the brake pedal spring to bring the pedal with grommet up to the bottom of the floor. With the master cylinder attached to the frame, screw the clevis to a position where you can insert the clevis pin through the brake pedal, clevis and insert a cotter pin. Doing the connection this way puts no pressure on the link and keeps the piston in the proper position. Now with a vise grip on the serrated portion of the link, tighten the link nut.  Bill