What Ed Said.
Chevy kept using that part on the end of the pedal shaft even after they changed the brakes to Bendix from Huck in '51.
Here's a picture of it. You need to compress the spring a bit.
Brake Pin Lock
And if you need a new one. Check Here (You have to buy a pair). Be aware there are two different OD's available, so you should measure to be sure they match yours.

Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.