January 15, 2024

As a part of the vehicle’s hydraulic system, brake fluid should always stay contained. So when it begins to leak from the vehicle, you know that this symptom is likely indicative of a more complex problem. Your customer may have reported fluid leaks on their driveway, and when you check beneath the hood, you find fluid on the brake booster housing. Now what?

Today we’re going to be talking about some possible causes and solutions for a brake fluid leak on the rear of the brake master cylinder. Two of the most common causes for this leak are damaged seals on the master cylinder itself or an issue with pushrod alignment on the brake booster.

Damaged Seals

To determine if it’s an issue with the seals, thoroughly check all seal points for damage, corrosion, or compromise.
Depending on the age of the master cylinder, you may find that the O-ring seals have become dry or brittle and need replacing in order to restore that factory seal and stop the leaking.

Pushrod Alignment

If the seals on the master cylinder look good, then check to see if the brake input seal and pushrod are properly aligned. When the brake pedal is depressed, there is a lot of pressure applied against the pushrod from the brake fluid.

If the pushrod is not parallel to the piston rod, that lateral force could transfer across to the master cylinder, causing leakage past the rear piston seal and out of the rear master cylinder seal. Misalignment or excess pushrod movement can create too much load on the master cylinder, resulting in an environment where leaks can occur.

Replace Master Cylinder

If you’ve determined that the master cylinder seals are good, as well as proper pushrod alignment and positioning, a third step to take is replacing the master cylinder altogether.

Ultimately, the customer’s brake master cylinder is leaking, and it shouldn’t be. So if your inspection of critical parts leads you to believe that this problem is occurring because of a larger issue with the master cylinder itself, replacing the part with an ultra-premium OE quality brake master cylinder may be the way to go.