Brake Mods Bias

This is a continuation of Brake Basics page. Click the button below to go back to it if haven’t read that part yet.



Brake Mods

Well now that we have covered how the brakes on a C4 Corvette work lets see if we can improve on what the factory gave us to work with.

As I mentioned at the start of all of this the C4 Corvette actually has very good brakes from the factory. But there are always areas that can be improved and if you start pushing your car to the limits you will need that edge.


Brake Bias

The C4 Corvette like most cars uses a proportioning valve to control how much pressure goes to the front brakes versus the back brakes. Without this valve you would experience rear brake lockup during hard braking and the nose of the car would dive downward excessively at the same time. Neither of which are ideal for controlling a car in a panic stop or when trying to squeeze the last second out of a tight road course.

To change how your car applies brake pressure you need to adjust the proportioning valve. The effect this valve causes is referred to as brake bias.



The valve is part of the master cylinder. In the diagram above parts 19 through 23 make up the proportioning valve. The part we will be changing for this mod will be part #21. This spring controls the brake bias.

So we know how to change the brake bias, but what do we change it to? This part is where it gets tricky. Like most off the shelf solutions its easy to make some bad mistakes here and its even harder to make a smart choice.

The stock setup in general is 85/15 for bias.

That’s 85% of the pressure going to the front wheels and 15% going to the rear. As you can see from this the rear brakes don’t do much with the factory bias setting.

Most off the shelf spring kits will give you 7% more pressure to the rear brakes. This means we can adjust the brake bias to 78/22. Which will make a very noticeable improvement in panic or performance related braking.

So which one to buy? The average price of this mod should run around $13 plus shipping. The DRM spring kit is very popular so I will link it in this article. There are other choices so I’m not going to say this is the only one to buy but this is one you can get.

DRM Brake Bias Spring


Once you have which ever kit you chose to purchase you still have to install it. Installation is pretty simple but will take a bit of time. Removing the master cylinder from car is recommended.

Start by placing something under the master cylinder to catch any brake fluid that will leak out of the master cylinder during the removal process.

Unplug the electrical connector from the master cylinder and set aside. Next remove the two brake lines from the master cylinder. Now remove the two nuts that hold the master cylinder to the brake booster. Slide the master cylinder off the booster and find a good work surface for the install.

Now remove the bolt from the end of the proportion valve housing on the master cylinder.

At this point we need to get the combination piston assembly out of the housing. You can strike the housing with a piece of wood or use a hammer to strike a piece of wood laying against the back of the housing to knock the valve forward to remove it.

Once removed wrap the valve in a piece of paper towel to keep from damaging it and place the valve (spring end up) into the end of a 19mm deep well socket. Next put the socket in a vice with the valve sticking upward.

Next using a snap ring pliers remove the c-clip that is holding the old spring onto the valve.  Note the orientation of the spring to the valve. Remove the old spring. Place the new tapered spring onto the valve in the same direction the old one was in.

Now things get fun. You will need to use a flat head screw driver that is wide enough to span the width of the spring. Press the spring down far enough to allow for reinstalling the c-clip. Place the c-clip around the shaft of the screw driver first. This way if the clip gets away from you while trying to install this you won’t have to call in a search party to find it.

Slide the c-clip into place with the snap ring pliers and release the c-clip.

Remove the valve from socket and paper towel wrap. Clean and lube the valve. Now reinstall the valve into the housing making sure that the spring side is outward. Install the new oring and the screw in cap back into the housing.

At this point you just need to reinstall the master cylinder back onto the car. Most bench bleed the master cylinder at this point since we introduced a ton of air into it removing the valve. So once you have done this place the master cylinder back onto the studs on the brake booster and tighten the two nuts back in place. Now connect the brake lines and the electrical connector. Bleed the brakes properly.

Congrats you now have a new brake bias spring installed.


I’m done right?

No other modifications are needed when you are working with the factory brake calipers and master cylinder. But what if you start doing other mods? Such as larger brake calipers? This does change things.

Basically the bias spring mod will not cause problems in itself. But the bias amounts will change. Larger calipers have either more pistons inside or larger piston bores. These hold more fluid. The pressure amounts are the same but you now need to move more fluid to achieve the same movement you used to cause when pressing the pedal. Rotor design an material and brake pad compounds also effect bias. Its impossible to tell you how much of effect this will  have since there are to many factors involved when other mods are done.

The main point is the spring is ideal for the factory setup. So what is the solution when you go beyond that? Well that depends.

There are two schools of thought on this and both are technically right. I will try to explain both camps rallying points  🙂

  • You need to use an external adjustable proportioning valve.

This camp believes that once your out of the area that the factory setup can handle you have to go external. Its easy to side with this since you are seeing a problem with brake bias that you can’t correct with a spring swap so external is the way to go. Plus you can adjust things to your hearts content since its adjustable.


  • You can adjust brake bias simply by pad choice, caliper and rotor size.

This camp believes that there is no point in replacing the proportioning valve since you can achieve the same results by properly designing your brake system. Since rotor size, caliper design and pad choice directly effect pedal pressure required and stopping power.


So which ones right? Not going there !

I will point out my opinions in the matter. I fall into the second camp. The adjustable external proportion valve is a cheap way to try to Band-Aid fix a braking system but the admittedly more expensive correct solution in my opinion is to build the entire system correctly to start with. Sizing you components to work together is the key to successful brake bias after changing the factory spring.

Whichever camp you decide to side with remember this. Cheap out on your brakes and you will find that hospital stay will cost you a ton more than it would of cost you to do the brakes right to start with.

The factory setup isn’t that bad and can be improved for under $15. If you need to go beyond that then make sure you can afford the price of admission. If not keep it stock till you can.


Next Up Brake Mods – Brake Hoses


Brake Hoses


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