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Disk Brakes FAQ
Why are disc brakes better than drum brakes?
Disc brakes are not subject to fade from heat or water. Disc brakes also do not drift out of adjustment.
Which calipers are better - 4 piston or single piston?
Single piston calipers are now the standard. They are much more reliable since there's only one piston which could possibly leak...not four.
Do disc brakes require more pressure than drum brakes?
How much pressure is needed for disc brakes?
You will need at least 1000 psi to the front wheels.
Can I use my drum brake master for disc brakes?
No, for two reasons. First you will not have enough pressure or volume and second the drum brake master will have a residual valve that will cause the disc brakes to drag.
Can I run disc brakes with a manual brake master?
Yes. The bore size of the master can not be larger than 1" and your leverage at the pedal has to be greater than power brakes.
If I add disc brakes will I need a proportioning valve?
Yes. The addition of a proportioning valve is a must. Without it your braking system will not operate properly.
Can I re-use the old drum brake valve when I change over to disc brakes?
No. You must use a disc/drum valve.
I have converted my car to disc brakes and now I can't get a pedal. It goes to the floor no matter how much I bleed it.
Check the bleeder screw orientation on the calipers. The bleeder screw must be on top in the 12:00 position. If it is not you will not be able to remove all the air from the system.
Master Cylinders FAQ
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 my manual master on a booster?
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.
Power Boosters FAQ
Is a power booster required with disc brakes?
No. Although a power booster gives a good pedal feel, manual brakes work fine.
What are the symptoms of a bad power booster?
A bad power booster will give a very hard pedal, it will feel like you need two feet to stop the car.
What does a booster do?
A power boooster helps assist pushing the master cylinder piston when you apply the brakes.
How much pressure should I be getting to the wheels with a power booster?
Typically you should expect about 1000 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" booster that puts out only 900 psi pressure. Use our AC2004K Pressure Tester Kit to get a pressure reading at any point in the braking system.
What size booster do I need?
For lighter weight vehicles like street rods a 7" booster will work fine for disc/drum set ups. For a 4 wheel disc system a 7" booster will not give enough assist. An 8" dual diaphragm booster is the ticket for 4 wheel disc.
Can I use a Ford Booster on a GM vehicle?
If you can mount the booster properly and link it to the pedal then any booster can be used on any type of car.
How much vacuum is needed to operate a booster properly?
For a power booster to function properly you will need at least 18" of vacuum at idle in park. Anything lower will give you a hard pedal.
I have power drum brakes, can I use the same booster if I change to disc brakes?
Yes. Just be sure to use a disc brake master cylinder.
What happens if the vacuum is too low?
If your vacuum level is too low you will experience a hard pedal and it will feel like the vehicle won't stop. Install our AC9001K Electric Vacuum Pump to maintain a constant 20" of vacuum.
I have a radical cam. Is there any way to supplement my low engine vacuum?
Yes. We offer the AC9001K Electric Vacuum Pump that works off the 12 volt system and gives a constant 20" vacuum.
Will a reserve canister help?
Somewhat, but usually they will only show a moderate increase in braking performance since the engine doesn't really get to a sufficient vacuum level on a constant basis. You really need a constant 18" at least.
What are my options if I don't want to use the vacuum pump?
Not many. Without the supplemental pump the only choice is to eliminate the booster and go to manual brakes.
How will I know if my booster is bad?
A bad booster will give a very hard pedal and it will feel like the vehicle will not stop.
How do I check to see if my booster is operating properly?
Shut off the engine. Depress the brake pedal a few times to evacuate the booster. Apply a steady pressure to the pedal and start the engine. The pedal should fall slightly.
Is there any way to decrease booster size and still maintain assist?
Yes. By using a dual diaphragm design booster you can maintain a high level of assist while keeping a small size. MP offers an 8" dual diaphragm booster that fits most street rod and GM applications.
I have installed a big block and the booster hits the valve covers. Is there anything I can do?
We offer an 8" dual diaphragm booster that will work for most GM and Ford applications.
Will I need a special booster with 4 wheel disc brakes?
The main consideration with 4 wheel disc brakes is that you have plenty of power assist. Don't undersize your booster with a 4 wheel disc set up.
When I step on the brakes the pedal feels good but I get no braking. What could cause this?
One of the causes of this 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.
I have installed a new booster on my car and now the brakes drag. Why?
Again this can be due to a mismatch between the booster pin and the master cylinder piston. Too long a pin will cause this.
I have installed power brakes on my origional manual brake car and now the brakes are extremely sensitive. Why?
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.
Power boosters keep going bad on my vehicles. Why?
You are probably getting corrosive vapors back into the booster. Install a vapor trap and that should eliminate the problem.
Can I rebuild my own booster?
This is not a good idea. There are many parts inside the booster that will require special tools to assemble and re assemble.
What does a proportioning valve do?
A proportioning valve is used in the rear to decrease the rate of pressure rise to the drums relative to the pedal force as weight is shifted to the front during braking. This prevents the rear from locking up under hard braking conditions.
What are the symptoms of a bad proportioning valve?
If your prop valve has gone bad your rear wheels will lock up easily, especially on wet surfaces.
What does a metering valve do?
A metering valve or "hold off valve" is used in the disc portion of a disc/drum system to hold off the application of the front discs slightly allowing the slower reacting rears to catch up. This provides rear stability on wet surfaces and reduces excessive pad wear.
What are the symptoms of a bad metering valve?
Your car will nose dive and the front pads will wear too fast.
What does a residual valve do?
There are two different uses for residual valves. The 10 lb valve is used to hold a residual pressure to the drum brakes to keep the shoes out close to the drums giving a higher firmer pedal. The 2 lb valve is used in the disc when the master cylinder is lower than the calipers to prevent back siphoning of the fluid from the master.
What are the symptoms of a bad residual valve?
The brakes will be very spongy and you will need to pump the pedal to get good brakes.
What is a combination valve?
A combination valve incorporates metering and proportioning into one valve providing all necessary valving for disc/drum systems.
Should I use an adjustable proportioning valve?
Not if you can help it. The adjustable valve will only provide the proportioning function and not the metering that is needed.