Shallow Vs Deep Pocket Master Cylinders

[rn2fine]

I have a question related to trucks and Suburbans made around 1972 with disc brakes. I have noticed recently in a catalog that master cylinders come in both what is described as a "shallow pocket" and "deep pocket". The description seems to refer to the depth of the pocket, 1/4" or 3/4", that the booster rod fits into. I ask because when I bought my 72 I immediately put a new master cylinder on it. The pedal nearly touches the floor when braking since I installed the new master cylinder. I am wondering if the original master cylinder was a shallow pocket but the replacement I put on was a deep pocket causing the pedal to travel further to achieve braking. Does anyone have knowledge or experience about this issue? Thanks !!!

[cparman]

did you match the old master to the new one? GM used a Delco or a Moraine brake booster in those years. Don't remember which was which ( old fart ), but I think the place to start is to make sure the old and the new look the same. After that you should check the clearance between the booster rod and the master cylinder.

Did you bench bleed the master?

do the brakes apply, even when the pedal is on the floor?

I have experienced low pedal a bunch of time, and this is my simple way to check clearance:

Unbolt the master, without disconnecting the lines. Take a small ball of clay, 1/4" or so. push into the back of the master cylinder piston. bolt the master back to the booster.DO NOT DEPRESS THE BRAKE PEDAL. Unbolt the master and see if the clay squished? you need a bit of clearance 1/16" or so. If not there is too much clearance. Some booster rods are threaded, and you can unscrew them to get the clearance you need. if it is not adjustable I will take my welder and make the rod longer.

The difference between a Delco and Moraine is a ton! The difference should be obvious. HTH

[robnolimit]

Here is a fix that we use from time to time. The bore wil be appx. .445". calculate/measure the difference/gap from the booster rod to the M/C piston bore bottom, and then cut a piece of a 7/16" bolt or cold rolled rod to fill the gap. It will be appx. 5/8" long. Slide the spacer into the master, check the fit, and go go go.