coppertopgmc

The master cylinder on my 71 cutlass is bad, my car has 4 wheel drum brakes and I am going to eventually convert to front disc, would it be an issue if I put a disc/drum master cylinder on it now with four wheel drum brake System?

joe_padavano

Funny you should ask this, as I just did exactly this on my 62 F-85 before Power Tour.

From a functional standpoint, the wheel cylinders neither know nor care what the M/C looks like. All the wheels care about is the diameter of the master cylinder bore and the stroke. I wanted to convert my 62 to a dual circuit M/C, but the configuration of the car requires the M/C outlet ports to face the engine. Also, since this is a manual brake car, I wanted a smaller M/C bore than the stock 1". The commercially available manual brake M/Cs that meet that criteria are severely limited. I was pretty much stuck with the Fox-body 7/8" bore disc brake unit. I have drum brakes. The only functional difference is that the outlet ports for drum brakes will have a residual pressure valve built in; those for the front discs do not. I went without, figuring that if I needed the RPV, I could add an aftermarket inline unit. The brakes in this car have never felt better. I just drove 2600 miles, first on Power Tour, and from there to Lansing, and back home. Not one issue with the brakes. This includes the winding parts of US58 that we took to get from Martinsville to Bristol.

In your case, pay attention to the bore of the disc brake M/C. If it's the same as your drum M/C, you'll have no issues. If it's larger, you'll have a hard pedal.

coppertopgmc

Thanks Joe, I know the master cylinder on the car is a1inch bore and the front disc is 1 1/4 so I’ll have to decide what to do, funny thing is I had very little brake pedal and the light would come on, the back brake would lock but not much front brakes, then yesterday I went out and the pedal pumped up and now the front brakes are working and no brake light so I don’t know what’s going on?

joe_padavano

It sounds like you have an internal leak in the master cylinder, in the seal between the front and back pistons. That's an indication that it's time for a new or rebuilt M/C.

coppertopgmc

Thanks Joe, I guess I better get it replaced soon if I’m going to drive it much this summer, I’d hate to lose them and cause an accident!

joe_padavano

Well, to be honest, a drum brake M/C for your car is about $21 (plus core charge) at RockAuto. It's probably easier to just use a correct replacement for now and change the M/C when you do the disc swap, depending on your timeframe for the swap.