For those of you who have done a brake upgrade, what master cyl have you been using? Mine is currently weak and I am planning an upgrade, I cannot find a stock replacement for less than $140. What else is out there that I could use, planning a brake upgrade, that would be cheap or that I could find rebuild kits for? Or do you guys know where I could find a rebuild kit?

I think you need to get the '79 or older since the '80 and up were disc/disc setups. Double check the years. Garry brought up the 620 m/c ('78 and newer) and I found that they were just a bit cheaper too. The difference between them is the bore size....I'd have to look to see which is a match.

Since a big part of this forum is for education, I'll add this, but I'm sure most of the "big" guys on here already know this.

The real difference between the disc/drum m/c of the older 280's and the disc/disc m/c's of the newer, is that the drum portion of the m/c has (is supposed to have) a residual valve built into it to keep the shoes closer to the drums. It is supposed to eliminate having to pump the pedal to get enough fluid into the slave cyl's to engage the shoes. If you have your shoes adjusted correctly (which would be close to the drum), I doubt you'd notice a difference.

Since disc calipers stay very close or even partially engaged on the rotor, there's no need for the residual valve.

The bore size difference between the 620 m/c and the 280ZX m/c is minimal. The 280 has a bigger bore which means that it pushes more fluid into the line when pressed the same distance as a smaller bore m/c. Since you will essentially have less "leverage...so to speak" on the fluid, the pedal will tend to feel a bit heavier. The smaller bore will have a bit more pedal travel, but will be slightly easier to push. It's pretty much all relative anyway. When I started driving my wife's 521 with the discs up front and a '78 280ZX (disc/drum) m/c, the pedal felt pretty heavy. Now that I've driven it daily for over a month, it feels normal.

If anyone wants to learn more about the brake system, I think I can dig up some really good websites on it.   

If you have or are planning on doing an upgrade to the 280ZX brakes on your 510, get the 280ZX m/c to start with. The calipers will require more fluid so the bigger bore will give you a better set up.

If you do go with the 280 m/c....let us know if the pedal is noticeably firmer or if it feels the same.

For any of you guys with pickups, the 620 m/c(73-77) is a great way to upgrade to a dual circuit.

I bought mine at Knecht's....if I remember right, they had the 280ZX m/c on the shelf. I'm thinking the 620 one was too, but even if it wasn't, it was a same day order.