Master cylinder

[Tim P.]

on the master cyl, in your pic is the front resivour for the front brakes or the rear ? What is the proper setup for this as to small res. and large res to front and back ? Thanks Tim.

[FORDification]

I don't know what picture you're referring to, but typically the rear reservoir goes out to the front brakes and the front reservoir goes out to the rear brakes. An all-drum system will have two equal-size reservoirs, while the disc/drum master cylinder has a larger reservoir for the front brakes because the disc brake pads wear faster than the drum shoes and the fluid reservoir will drop faster.

[Tim P.]

Ok thanks. Does that effect the performance of the brakes stopping power as to front and rear ratio? ie, the amount of braking power from front to backon the system? Tim.

[FORDification]

A larger reservoir (or larger diameter brake lines) don't mean more pressure, simply more volume. Here's some interesting info I saved discussing master cylinders.

TYPES OF MASTER CYLINDERS:

DRUM / DRUM - A drum / drum master is designed to deliver fluid pressure and volume to the front and the rear of a braking system in equal proportions. Drum brakes will require less fluid and pressure than disc brakes. Typically a drum brake master will be smaller than a disc master and the fluid reservoir chambers will be equal in size. Since drum brakes require the use of residual pressure valves the original drum master cylinders had residual valves built into the outlets. Later model aftermarket units do not have these valves and they must be installed in the lines externally. Failure to incorporate residual valves will cause spongy brakes. Generally speaking it's not a good idea to use a drum brake master for disc brakes since the amount of fluid the cylinder pushes will not be sufficient.

DISC / DRUM - A disc / drum master is designed to push more fluid volume to the front disc brakes since disc brakes require more volume than drum brakes. Generally speaking a disc / drum master will have one fluid reservoir larger than the other.This is because the disc brake pads wear faster than the drum shoes and the fluid reservoir will drop faster. Original disc / drum masters had a built in residual pressure valve to the rear drum brakes only. Never reverse the outlets on a disc / drum master. Doing this will cause the front disc brakes to drag excessively from the residual pressure valve.

DISC / DISC - A four wheel disc brake master cylinder is designed to supply more fluid pressure and volume to the rear disc brakes than the disc / drum master does. This is acheived through an internal piston re design. The piston that feeds the rear brakes on a disc / drum master will run out of stroke, limiting the amount of fluid pressure and volume that may be supplied to the rear isc brakes. The four wheel disc master re design delivers the extra needed volume and pressure to the rear allowing your rear disc brakes to function properly. If you attempt to use a disc/drum master on a four wheel disc system you will get poor rear brake function and experience a spongy brake pedal with a long pedal travel.

[Tim P.]

Well Keith I have the drum/drum system but the master cylinder is a disc/drum type it comes with the brass adapters for the lines are these the residule fittings your talkin about and will this hurt the braking using this type master cyl without the brass fittings it lookes the same as the one you post earlier for me on the clearance of the booster on your truck. Thanks Tim.

[FORDification]

You most definitely do NOT want to use a disc/drum master cylinder on an all-drum vehicle. Each reservoir on the disc/drum MC has a different size line, which is probably why a previous owner 'adapted' it to fit your truck. I'd strongly suggest replacing it with the correct unit before doing any driving.

[Tim P.]

Keith I've been running this style since I owned this truck and the brakes have not presented any unusal performance issues as lone as i maintain the system ie. replacement parts ,shoes,springs etc. I don't get it?????

[68F250]

Tim, here's the deal. Some drum/drum M/C's had equal volume reservoirs but the majority of them have a big/little just like a disc/drum M/C. I guess they figured out that the same casting could be used for both. The big difference is in the bore diameter. A drum/drum M/C has a bore diameter of 1.00" and the disc/drum one is 1.25". Also, a drum/drum M/C will have two of those brass fittings on the outlets, they are the residual valves. A disc/drum M/C will have only one brass fitting on the front outlet (that goes to the rear drum brakes). Hope this helps.

[Tim P.]

Thanks for clearing that up for me as mine only has 1.00 piston and two brass fittings but uses the same casting as the disc/drum M/C that was really had me bewildered for a minuet as i have aways used the newer style m/c Thank You Tim.

[68F250]

Tim, here's an update. I just replaced the M/C on my '68 2wd w/ drums and also added a power booster I've had laying around for a few years. The new replacement M/C came with two brass adaptors and that's all they are, adaptors. The residual valves must be in the M/C, I'll have to look at a diagram and see where they are. If you buy an aftermarket M/C, they will provide the residual valves separate as needed and they look like the brass adaptors. Just an FYI, the M/C for drum brakes are all the same, manual or power. The power booster I used was the tandem diaphragm one and had the non-adjustable rod. The length was perfect and put the brake pedal exactly in the same spot as it was before. Works great but now I have to relearn how to drive this thing without locking 'em up!

[flyboy2610]

My '68 F100 has drums all around and it too has the unequal reservoirs.
I gues that is one less thing to worry about when I convert it to front disc brakes.

[FORDification]

The question is: even though the master cylinder has unequal reservoirs, does it have the proper residual valves? They're very different for drum vs. disc. From what I've heard, the drum/drum master cylinder has a pair of 10-pound residual valves, whereas the disc MC has a 10-lb for the rear brakes and a 2-lb. for the front. The 2-lb valve won't give you enough pressure to properly actuate the drums.

[Tim P.]

Well mine has only the brass adapters as far as the res, valves im not sure now if they are built in or not sure would like to know as some of the auto parts people are clueless to the job they do and the service they should provide any more input on identiflying the difference or how to reconized if they have valves internally or not. Thanks Tim.