1973 Master Cylinder Mystery

[Father&son56project]

My son just picked up a 1973 C10 longbox which is now at my place to get prepped for a safety check. The truck shows evidence of having been monkeyed around with by some previous owner (different engine, poorly done electric rad fan, etc.). It has front disk, rear drum manual brakes. I don’t know how it rolled off the assembly line (RPO sticker is gone and build sheets are nowhere to be found).

Here’s the issue: The MC (which is cruddy and leaking) has 2 equally sized reservoirs and a casting number of 5488115. When I look this up, I find very few references, and they are all for camaro/firebirds from the 60’s. Given the monkey business elsewhere on this truck, I have to wonder if this is a wrong part (I need to replace it anyhow though).

When I go online to rockauto, partsavatar, etc. the 1973 C10 MC’s ALL have 1 large/1 small reservoir, a 1 ⅛ inch bore and they ALL have a dimpled recess for the pushrod to sit against (input from this site shows that the manual MC’s have a deep hole rather than a shallow dimple).

When I go onto the LMC site they show a unique MC meant for manual brakes, but only for 1974 and up. The same goes for Rockauto when I search for 1974 models (they have several with a 1 inch bore and a deep recessed hole).

If I search under the 68-72 years, I again find deep bore MC’s for manual disc/drum brakes. It’s only 1973 that is a problem. So, after my long winded description I have 2 questions:


1) Has anyone on this site replaced their 1973 manual brake MC (and if so, what make & model did they order)

2) In the event that nobody can answer question #1, is there any harm in using a manual MC from a different year (as long as its for the same light duty braking system of course)

I look forward to your input as this really has me scratching my head!

[MJN]

Looks like there were two master cylinders used on C-10 in '73 per the parts catalog. Also, standard rear brakes were 11"x 2". These part numbers should help with a starting point at least with finding the correct master cylinder.

[Father&son56project]

MJN: Thanks for that pic out of the parts catalogue! I looked up those numbers and there are readily available master cylinders that claim to interchange with those numbers. Curiously though, they all have the 1.125 inch bore and a shallow brake rod dimple (which is exactly what various brake companies say not to use with manual brakes). After seeing your reply I got curious and took a closer look at the MC on my son’s truck. While it was pretty cruddy, I managed to get it cleaned up and flushed out in order to give it a try. I also looked carefully at it, and it has a 1 inch bore (as is recommended for manual brakes) AND a shallow dimple (as is not recommended for manual brakes). Mind you, god only knows if this is the correct MC so I’m not basing any decisions on it. I got awful close to being able to test it, then discovered that the front drivers flex hose was plugged so I couldn't finish bleeding the brakes. If I do decide to change it, I will probably just get one meant for a ‘74.

Thanks for the help!