SPD Tuning Service

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Why does my Subaru have a soft brake pedal?

Common question that has three or four different answers. Let's assume the car is fairly new and nothing is suspected wrong, but...well, the brake pedal is just too soft.

Subaru, along with most other car manufactures, uses a dual stage vacuum booster on the brake master cylinder. It is intended to satisfy the majority of men and women who do not want a car that requires heavy effort to control. For this reason the Subaru steering is kept light and the transmission shifts with ease.

Here is how it works: The brake pedal is depressed perhaps an inch and the car stops medium hard. Simple physics tells you that if you press twice as hard on the pedal the car line pressure will go up by only the square root of that increase in foot pressure.. This means that as you get into the brake pedal it should get rapidly harder and harder as the car stops more quickly. But the Subaru pedal stays soft. What is going on?

The dual stage vacuum booster has one level of assist in the first part of its stroke and as the brake pedal is pushed in, a port is opened on a second, much more powerful boost stage then compensates for the large increase in actual brake line pressure. The pedal stays relatively soft even as the ABS kicks in.

When driving the car hard, a double clutch down shift is done with the ball of your foot working as the pivot on the brake pedal as you heel, or the side of your foot presses and releases the gas pedal. Many drivers are finding that this soft brake pedal inhibits there ability to modulate both the brake and the throttle smoothly. How do we fix this problem?

SPD Tuning Service offers a single stage booster for the Subaru that replaces the larger dual stage booster. The pedal becomes remarkably firmer and easier to read, while still keeping the relatively light control inputs that are a Subaru characteristic. This is especially true when pushing deeply into the brakes during high performance driving. You gain a firm place, the brake pedal, from which your foot can rev the engine for smooth take-up of the drive train during critical braking maneuvers and even more important your foot can roll off the brake pedal and into the gas during the turn in down to the apex of the corner. The ability to get a smooth transition of weight off the nose of the car during turn-in is the hallmark of a smooth, quick driver. To achieve this one needs a proper seat and pedal combination. The pre-1999 cars use the booster kit alone. Unfortunately, the 1999 and later cars have a redesigned master cylinder/booster that is not interchangeable. We can still provide the single stage booster kit, but 1999 and later cars will require a new master cylinder at an additional expense.

Lets assume that you are at track day and have a low brake pedal the first time you touch the brakes after each corner, but the second time the brakes work fine. What is going on here?

You could have air in the line or it could be possible that you have a worn wheel bearing. Bleed the brakes. Still the same? read on.

In order to save costs, Fiat many years ago developed a disk brake caliper that had a piston on only one side. Up to that time all disk brake calipers had opposed pistons. One or more on each side of the disk. All passenger car manufactures have adopted this design with only a couple of exceptions, one of which is Porsche.

When a wheel bearing flexes (and they all do to a lessor or greater degree) the disk will work against the pads and try to open the pads up. An opposed piston caliper will follow the disk, while a single side piston design will allow the pads to be pushed away from the rotor. With severe wheel bearing flex you will push the brake pedal down 1/8th to 1/2 the pedal travel just brining the pads back into contact with the rotor.

This is actually very uncommon in a Subaru, because they have the an unusually wide distance between the baring pairs in the hub. Ever wonder why Subaru wheels have such huge offset? Now you know. My car with 75,000 miles is just showing enough play that the pedal is a bit low after a really hard corner. If you tap a curb or hit a really big hole it is possible for pad knock back to develop. How much play is too much? Flexing the hub by hand the face of the rotor need only move .01" to cause a noticeably low pedal. Usually the maximum hand flex is .006". You do not have a wheel bearing problem that you can hear or one that is causing abnormal amounts of heat in the hub, just pad knock back. SPD Tuning Service offers teh 298mm 4-piston caliper kits remove this problem from the inventory as well as offer superior fade resistance.

Do you have air in the lines or moisture? A soft spongy pedal, that comes up after a stab or two but always comes back is a sign of air in the fluid. Always check the fluid level regularly. Bleed the brakes first if you have a soft or variable pedal. Brake fluid absorbs moisture, so I replace my fluid every 24-36 months. If you replace fluid every couple of years you should have a long service life out of the brake system. No particular need to use fancy fluid. I use Castrol LMA in pint cans to keep it fresh as possible. It is not a good idea to keep open fliud in quart or larger cans for an extended period due to the moisture absorbant nature of brake fluid.

Send mail to mshields@spdusa.com with questions or comments about this web site.
Last modified: May 23, 2000