Changing out the master cylinder on a 69 Satellite 383 car right now with manual brakes. Just got a new MC for it and apparently they do not include the rubber bushing for the push rod anymore, anybody have a source for these? Carquest came up with part number H520 but they said it is discontinued. You have to have one to install the MC and all the local parts stores are stumped. Might just make them go through all the master cylinders they can find to find one. My only option right now is to get a MC rebuild kit for $25, the MC itself was $35!

You can get a piece of rubber tubing from Home Depot and cut to length.
That's what I did

go back to your closest parts house & give them the pushrod & have them take it back to their hose rack & cut a snip of vac line that fits snugly over the rounded nub on the end. cut it to the same length as the groove in the pushrod. I've had a piece like this & one in the M/C box compared side by side & no difference except for a thin/shallow groove running lenghwise on the OE one which might be of no consequence & is produced during the manufacturing process or it is there deliberately to expell air so it seats all the way into the blind "thimble but in any case I've done several with the snip of line & it locks solid/no problems. On one or two I had to slightly sand the OD so it'd fit into the recess easier (tho you do want it pretty tight) as there are variations in some M/C's in the piston recess dia/taper & the snip goes on first then the pushrod/snip assy goes in the M/C.

So sounds like no off the shelf part, OK, looks like we will be making our own.

Mancini's used to have them. Worth calling them. If they do, buy two !!