Master Cylinder Question

Discussion in '1965 - 1973 Classic Mustangs -General/Talk-' started by Scrasauge, Jan 9, 2014.

  1. on my 65 someone already did what looks like a granda disc brake swap but how can i tell if the master cylinder is the correct one to use with disc brakes?
  2. A disk brake master cylinder has a smaller front bowl and larger rear bowl .the smaller front bowl is for the rear brakes and the larger rear bowl is for the front disks.the disk side of the cylinder usually has a check valve inside that a drum brake cylinder does not have . A drum brake cylinder is usually the same size bowl front and back .
  3. Ok going from my memory i think both bowls are the same size, if i need to change it out which one should i go with for a factory replacement?
  4. You may have a residual pressure valve in line ,if you do your current master cylinder will be ok. It will be about 3 inches long and a bout 5/8 in diameter probably red or blue ?if not you can add one . how do the brakes feel ?
  5. two possibilities;

    1: the master cylinder you have has been modified from a drum/drum system to a disc/drum system by removing one of the check valves

    2: you have an early disc/disc master cylinder from a corvette and a check valve was added inline somewhere.

    number one is most likely. if you need to replace the master cylinder, i recommend using one from a 75 granada, select manual or power brakes as needed. this assumes you have rear drum brakes by the way. if you do this swap, make sure you know which lines go where before you plumb the master cylinder into the system. it wouldnt do the mix the lines up.
  6. Im going to try to get a picture of what is on the car as soon as I can.
  7. In it's OEM configuration (as produced from the factory), any MC in an early Mustang with drum brakes would have had a residual pressure valve (RPV) under the seat, in the MC port(s) supplying fluid to the drums.

    If you go to a parts supply place to buy a replacement MC, chances are great that the replacement MC isn't going to have any RPV(s) installed in it, even if the new MC is a direct replacement for the OEM application.

    You can insert the shank of a small diameter drill bit into the hole in the MC port to see if a RPV is present. It's just a small duck-billed device. If it's present, the drill bit won't go very far into the port. If it's not present, the bit will go a pretty good ways into the MC port.
  8. ok sounds like when i decide to change it ill check that to see if it has the RPV