Tech Corner
Problem: Lucas Brake System Leakage on Rear Wheels
Solution: RC-01 Inline Residual Check Valve
For the past several years, KBR has been extensively involved with remanufacturing Lucas brake system components. We offer every component related to both the brake and parking system from the brake pump, hydromax, booster, master cylinder, hydraulic relay, dash control, calipers, individual wheel cylinders and spring brakes. Our most popular version of the rear brakes is the complete back plate assembly, available with or without brake shoes.
The most common complaint is that this system is prone to leakage. In fact, with literally thousands of cores returned with brake shoes, I don’t remember ever seeing a pair of shoes that were worn out! They are just saturated with brake fluid (or gear oil in the case of wheel seal failure). The vast majority of warranty returns are found to have very small scratches on the cup that allow brake fluid to pass between the cups and the cylinder wall. While these scratches are determined to have been caused by contaminated fluid, it raised question as to why such small scratches would cause the leakage.
Upon testing these returns, we would pressurize the cylinders with 1200 PSI of nitrogen gas and were unable to detect any leakage. This prompted us to think that perhaps the leaking was occurring at low pressure. With that in mind, we enlisted the help of a company that has the ability to not only do a sustained low pressure test, they were able to incorporate vibration at varying frequencies as well. The test pressure we used initially was approximately 10 PSI (+/- 3 PSI). This pressure was chosen because our experience with cup type drum brake systems was that 10 to 14 PSI was maintained to keep both the brake spring hardware from collapsing the wheel cylinder pistons in the bore and to keep the sharp lip of the cup expanded tightly against the cylinder bore.
The test results were negative, no leak at low pressure. Then, by chance, the employee that normally tests the sleeved cylinders prior to assembly noticed a very slight trace of tiny bubbles (sleeves are tested by submerging the castings in water and applying 1200 PSI gas pressure) after relieving the cylinder of all pressure. Like all wheel cylinder cups, the cup in the Lucas cylinder is of relatively hard durometer and relies on the very edge or lip of the cup to seal. Now we think we are on to something. It must be a defective residual check valve in the system causing all the leakers, right? Wrong. Since we remanufacture the master cylinders originally produced by Bendix, we knew that there was no check in the master, so perhaps it was part of the distribution (or proportioning) valve. We investigated that possibility to a dead end. So where is the elusive check valve that we know must be in the system to function properly? It does not exist until now. It seems that of the three manufactures involved in the design of this system, none included this Important little component. While the Lucas cylinders are “bottoming” self adjuster type, meaning they do not need to rely on residual pressure to keep the brake springs from collapsing the pistons, it is still a cup type seal and will truly benefit from this addition.
Further testing revealed that it takes approximately 25 to 35 PSI of hydraulic pressure before there is any movement of the brake shoes. So, the addition of a residual valve to maintain pressure on the cups to enhance sealing seemed logical. Using a known leaking cylinder, we first tested using a 3 PSI valve and experience noted improvement. From there, we continued to experiment with varying pressures until we found what we believe to be the optimum pressure. Once that was accomplished we were able to produce an easy-to-install piece that simply replaces an existing tube union.
Field testing, including tests with numerous school bus fleets, has yielded positive results. While not designed or intended to replace worn-out cylinders, this check valve has the potential to dry up systems with minor seepage. (Note: always inspect friction material and replace when necessary.) If you have experienced leak problems with this otherwise well-designed system, try our RC-01 valve and get results. With nearly 20 years experience in medium / heavy duty brake systems, you can rely on KBR for solutions.