Master Cylinder Q&A

Q.How does a disc brake master differ from a drum brake master?

A.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 a 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.

Q.What will happen if I use a master cylinder for drum brakes and it doesn't have a residual valve?

A. You will have a spongy pedal and you'll have to pump the pedal to get good brakes.

Q.I have manual brakes and I have an extremely hard pedal. Why?

A.Check the bore size of your master cylinder. If it is larger than 1 inch, then you'll have a very hard pedal.

Q.What bore size do you need for manual brakes?

A.If you use anything larger than 1 inch, then you will have an extremely hard pedal. The smaller the bore the more pressure that is output to the wheels.

Q.What is the difference between a power brake master and a manual brake master?

A. A power brake master will have a larger bore diameter than a manual master.

Q.Can I use my manual master on a booster?


Q.Can I use a power brake master without a booster?

A.Usually not. The bore size will be too large, resulting in a hard pedal, and the piston hole will be shallow, allowing the push rod to fall out.

Q.Should I buy a rebuilt or new master?

A. Always try to use a new master. Rebuilt units tend to have a high failure rate. The best way to rebuild a master is to sleeve it with stainless steel.

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

Q.How can I tell if my master cylinder is bad?

A.You will have very spongy brakes. Also, when you hold your foot on the brake pedal it will sink slowly toward the floor.

Q.Why should I eliminate my single piston master?

A. Safety. By going to a split system the possibility of a complete brake failure is virtually eliminated.

Q.Can I rebuild a master myself?

A. Yes, but you must have the correct rebuilding kit. Also, check the bore to be sure it is not pitted or corroded.

Q.I purchased a replacement master and it does not look like the original. Can I use it?

A.Probably yes. Many aftermarket replacement masters will have a different casting look but be the same internally.

Power Booster Q&A

Q. What are the symptoms of a bad power booster?

A. A bad power booster will give a very hard pedal, and it will feel like you need two feet to stop the car.

Q. How much pressure should I be getting to the wheels with a power booster?

A. Typically you should expect about 1,000 psi to the wheels for a disc brake system. A disc brake system requires this amount of pressure so be careful when using a smaller 7-inch booster that puts out only 900 psi pressure.

Q. Can I use a Ford booster on a GM vehicle?A. If you can mount the booster properly and link it to the pedal then any manufacturer's booster can be used on any type of car.

Q. How much vacuum is needed to operate a booster properly?

A. For a power booster to function properly you will need at least 18 inches of vacuum. Anything lower will give you a hard pedal

Q. I have power drum brakes. Can I use the same booster if I change to disc brakes?

A. Yes. Just be sure to use a disc brake master cylinder.

Q. What happens if the vacuum is too low?

A. If your vacuum level is too low you will experience a hard pedal and it will feel like the vehicle won't stop.

Q. I have a radical cam. Is there any way to supplement my low engine vacuum?

A. Yes. You'll need an electric vacuum pump that works off the 12-volt system and gives a 21-inch vacuum.Q. Will a reserve canister help?

A. Somewhat, but usually they will only show a moderate increase in braking performance because the engine doesn't really get to a sufficient vacuum level on a constant basis. You really need a constant 18 inches at least.

Q. What are my options if I don't want to use the vacuum pump?

A. Not many. Without the supplemental pump the only choice is to eliminate the booster and go to manual brakes.

Q. How do I check to see if my booster is operating properly?

A. Shut off the engine. Depress the brake pedal a few times to evacuate the booster. Apply steady pressure to the pedal and start the engine. The pedal should fall slightly.

Q. Is there any way to decrease booster size and still maintain assist?

A. Yes. By using a dual diaphragm design booster you can maintain a high level of assist while keeping a small size. An 8-inch dual diaphragm booster will fit most street rod and GM applications.

Q. Will I need a special booster with four-wheel disc brakes?

A. The main consideration with four-wheel disc brakes is that you have plenty of power assist-don't undersize your booster.

Q. When I step on the brakes the pedal feels good but I get no braking. What could cause this?

A. One of the causes is a mismatch between the booster pin length and the depth of the master cylinder piston hole. Be sure the hole is not too deep for the booster pin.

Q. I have installed a new booster on my car and now the brakes drag.

A. Again, this can be caused by a mismatch between the booster pin and the master cylinder piston. Too long of a pin will cause this.

Q. I upgraded my manual brakes to power brakes and now they're extremely sensitive.

A. The most likely cause is the pedal ratio is wrong. There are two attachment points on the brake pedal. For power brakes you need to use the lower attachment hole. Using the upper hole will make the brakes too sensitive.

Q. Why do power boosters keep going bad on my car?

A. You are probably getting corrosive vapors back into the booster. Install a vapor trap and that should eliminate the problem.

Q. Can I rebuild my own booster?

A. This is not a good idea. There are many parts inside the booster that will require special tools to assemble and re-assemble.

pedal ratio bore size lbs input psi out
6:1 1 1/8 75 453
6:1 1 75 573
6:1 7/8 75 748
5:1 1 1/8 75 377
5:1 1 75 477
5:1 7/8 75 623
4:1 1 1/8 75 302
4:1 1 75 382
4:1 7/8 75 499
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IA  52358
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