Rear disc master cylinder question

Discussion in 'The whoa and the sway.' started by duke350, May 15, 2020.

  1. duke350

    duke350 Well-Known Member

    My 72 GS has disc up front and drum in the back. I’ve seen several kits to upgrade to disc brakes all around but most don’t come with a power booster and master cylinder. Is that component required to replace if going from drum to disc in the rear only? Sorry if this has been covered. I searched but didn’t see this question answered. Cheers gents!
  2. duke350

    duke350 Well-Known Member

    Another question, some kits have 2” drop spindles for the front. I understand this to mean it would lower the front of the car only. Seems counter intuitive unless the “shackled up” look is what you’re after. As I mentioned my car already has the disc brakes up front and I assume I wouldn’t need those parts. Am I on track here?
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Not sure about the master cylinder, some have residual check valves for the drum brake section. That would cause drag with disc brakes.
  4. fishwater

    fishwater Well-Known Member

    I’ve been researching both of these questions lately. I’ve yet to see anyone offer a master for rear discs, seems like it is handled with the proper proportioning valve. Makes sense since the large fluid reservoir of the master handles the front brakes which need the most volume, the rear brakes need less fluid volume regardless of drum or disc, the bias of front to rear braking is most important.

    As far as drop spindles they do in fact only lower the front of the car so you will also need to change out the rear coils to level the car, there are a few suppliers who offer lowering springs. The benefits to the drop spindle is changing the front end geometry, using different ball joints (they make different height ball joints which help your front geometry) retaining the factory length spring for a better ride & adding disc brakes to a drum equipped car. Not all drop spindles change geometry, some are just a copy of the current spindles but there are others that are known as tall arm spindles which help bring the handling more in line with current vehicles by changing the camber curve. Hope this helps.
    duke350 likes this.
  5. Brent

    Brent Founders Club Member

    I use a master cylinder for a C2 or C3 Corvette, depending on which size piston I want. The vettes were set up for 4 wheel discs and have the correct residual check valves. They also have the same bolt pattern and pushrod depths as the factory MC so they bolt up to the firewall for manual or the original power booster. They seem to work well for me. The only downside is they are cast iron, so they are heavy compared to a modern aluminum one, if thats a concern.
  6. newmexguy

    newmexguy Well-Known Member

    The A cars back in the day used air shocks to lift the rear of the car. Shackles were on Novas and Camaros. The stock parts are fine, frankly don't understand the hype over tubular control arms and other off shore spindles and such.
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    I think you meant the LACK of residual check valves in a disc brake application. Residual check valves were used in drum brake applications to keep some pressure in the lines and speed up brake application. In a disc brake system, that would cause drag. Not sure about our stock MC's. In later years, OEM used cup expanders in the wheel cylinders to do the same thing.
  8. knucklebusted

    knucklebusted Well-Known Member

    If a 72 had original front discs, you will likely need a different proportioning valve. That's where the residual pressure is often found.

    I converted a 70 with the hold off valve front discs to 4 wheel discs by removing the hold off and left the basic distribution block on the frame. I did swap masters but I don't know if the original would have worked or not.
  9. duke350

    duke350 Well-Known Member

    Thanks for the help guys!
  10. telriv

    telriv Founders Club Member

    For the most part residual check valves are mainly used for masters that are under the car so fluid would not drain back to the master & have to pump the brakes when 1st. driving the car for the day.
    2PSI for discs & 10PSI for drums. They are available separately to be mounted in line. As stated some masters, not all, have them. I also used the 'Vette masters as they have the 2PSI built into them.

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