Master Power Brakes - Reference/FAQs: Master Cylinders

How does a disc brake master differ from a drum brake master?
A drum brake master will differ from a disc brake master in two ways. The amount of fluid that a drum brake master has to move is less than disc brake. Drum brake masters have 10 lb residual valves at the outlet to keep a residual pressure on the drums. If you use a drum brake master for disc brakes you would move an insufficient volume of fluid and the disc brakes would drag because of the residual valves.

What will happen if I use a master cylinder for drum brakes and it doesn't have a residual valve?
You will have a spongy pedal and you'll have to pump the pedal to get good brakes.

I have manual brakes and I have an extremely hard pedal. Why?
Check the bore size of your master. If it is larger than 1" then you will have a very hard pedal.

What bore size do you need for manual brakes?
If you use anything larger than 1" then you will have an extremely hard pedal.The smaller the bore the more pressure that is output to the wheels.

What is the difference between a power brake master and a manual brake master?
A power brake master will have a larger bore diameter than a manual master.

Can I use a shallow hole master in a manual brake application.
No. The piston hole must be deep so the manual push rod does not fall out.

Can I use a power brake master without a booster?
Usually not. The bore size will be too large giving a hard pedal and the piston hole will be shallow allowing the push rod to fall out.

Why is one fluid chamber larger than the other in a disc/drum master?
As disc brake pads wear the caliper pistons will move outward. The fluid level in the master will drop more rapidly than the drum portion so you need more reserve.

Should I buy a rebuilt or new master?
Always try to use a new master. Rebuilts tend to have a high failure rate. The best way to rebuild a master is to stainless steel sleeve it.

Can I use a disc/drum master for four wheel disc brakes?
Usually not. For 4 wheel disc brakes to function properly you will need a master that has a longer stroke.

How can I tell if my master cylinder is bad?
You will have very spongy brakes. Also when you hold your foot on the brake pedal it will sink slowly toward the floor.

Why should I eliminate my single piston master?
Safety. By going to a split system the possibility of a complete brake failure is virtually eliminated.

Can I rebuild my master myself?
Yes, but you must have the correct rebuilding kit. Also check the bore to be sure it is not pitted or corroded.

I purchased a replacement master and It does not look like the original. Can I use it?
Probably yes. Many aftermarket replacement masters will have a different casting look but be the same internally.

Descriptions

Your master cylinder is the heart of your braking system. In order for your system to function properly you must utilize the correct cylinder for your system's demands. The following brief descriptions should help you in determining what master you will need.

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 redesign 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.


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