Master Cylinder question

[zieg85]

I eventually will be converting my 67 Ford Custom to front disc brakes but I am not ready.  I find myself needing a master cylinder in the meantime.  Other than the 2 reservoirs holding a different volume of brake fluid is there any reason I can't install a disc brake master versus the 4 drum version?

[VileZambonie]

The conversion kits are pretty inexpensive. Why not git r done? Drum Drum reservoirs are the same size unlike disc/drum.

[zieg85]

The conversion kits are pretty inexpensive. Why not git r done? Drum Drum reservoirs are the same size unlike disc/drum.

I have a donor car for the spindles, rotors, calipers which are Kelsey Hayse 4 piston calipers which I have to redo.  I was going new master anyway, my question was would getting the correct master that I will be needing for now and continue using my 4 wheel drum brakes be a problem?  I would still like to enjoy the car before I tear it down for the upgrade.  Thanks Vile.

[ehjorten]

The question would be where are the residual pressure valves in your system? In a GM the residuals are in the master. If you go to a Disc/Drum master, you might not have the residual pressure valve in the primary circuit. This will mean that your front slave cylinders will not have the 10 psi or so to keep the lip seals in the slave pressed against the bore and they will leak. Usually the Disc/Drum master has a slightly larger bore to compensate for the larger piston area of the caliper. Larger master piston area will reduce the output pressure and shorten the stroke of the pedal. It will mean that you need to push harder on the brake pedal to generate the same stopping force. It won't be dramatic, but it will be different. Do you have power or manual brakes?

[zieg85]

The question would be where are the residual pressure valves in your system? In a GM the residuals are in the master. If you go to a Disc/Drum master, you might not have the residual pressure valve in the primary circuit. This will mean that your front slave cylinders will not have the 10 psi or so to keep the lip seals in the slave pressed against the bore and they will leak. Usually the Disc/Drum master has a slightly larger bore to compensate for the larger piston area of the caliper. Larger master piston area will reduce the output pressure and shorten the stroke of the pedal. It will mean that you need to push harder on the brake pedal to generate the same stopping force. It won't be dramatic, but it will be different. Do you have power or manual brakes?

Currently 4 wheel manual drum.  Planning on going to manual front disc/rear drum and changing out the differential valve to a proportioning valve at the time of the swap once all the components are rebuilt/replaced
 

[VileZambonie]

Disc drum uses a combination valve so make sure you get the correct one. I would run the drum brake MC until you are ready to do you swap.