Anyone with a 70s GM A Body with 4 wheel disc, upgraded their master cylinder to a more standard plastic reservoir from a newer vehicle?? I was wondering if anyone has done that and what did they have to do. I know master cylinders would have different pressure ratings for brakes so I'm not sure if it can be done with crazy modifications.

 

I expect several will quickly jump in and state that such a thing is not an "upgrade". And, well, they're not wrong. I just don't like the look of a rusting cast iron master, and pressure bleeding (my generally preferred method) is a rea PITA with that style. So it's just a change that I prefer.

I used the mid-80's Dodge D150 master for a long time. It has a 1 1/8" bore and the same fittings in the same layout. The mounting bolt holes are narrower (3.2" instead of 3.4") so you have to hog them out. There's also no residual pressure valves so you need to address that as appropriate. That was the one that was best for me, with my somewhat limited searching. It's also nice because they are readily available and rebuild kits are plentiful.

I recently started experimenting with the C5 master, but the fittings are a pain. They change mid-production, use metric threads, but 45deg inverted flare seats..... And it's a 1" bore.

 

Why do you think that's an "upgrade"? The reservoir is just a tank to hold brake fluid. The only thing the brakes care about is the bore diameter and stroke of the M/C. Changing the M/C bore without knowing what you are doing is exactly the opposite of an "upgrade", and swapping the M/C for one with the same bore is called a "replacement" with the same performance as before. The only "upgrade" would be if your old M/C was worn and internally leaking. A new replacement M/C would fix that.

 

I use the Eastwood Brake Paint. That solves the rusting cast iron problem and is resistant to brake fluid. There are several options for aluminum versions of the standard GM iron M/C. Speedway Motors has them, among others. The pressure bleeder cap for the GM iron M/C existed long before the ones for the plastic reservoirs, though frankly I find vacuum bleeding to work much better and with much less mess. And M/C bore changes let you adjust brake performance. Swapping from the OEM 1" M/C to 7/8" on my 62 F82 with manual brakes made a world of difference in brake performance and required pedal pressure.

 

That's how I feel about the old ones. Plus I hate having to get a screwdriver to pop the covers off. I might give a look into that Dodge one.