BMW and MINI, Power Brakes Weak, Hard Pedal, at Cold Start, Stop and Go Driving
I have a 2003 530i. The power brakes don’t seem to work well when I start the engine in the morning or after work. The pedal is harder than normal and I have to press it harder to get the brakes to stop well. I have to drive it a bit and then the power assist seems to come back and all works normal for the most part. Sometimes this also happens in stop and go driving, but once I get moving again the brakes are fine. The shop has replaced the brake pads and the vacuum booster (wow $$$) and bled the brake fluid multiple times, but the issue has not changed. Do you have any suggestions?
The symptoms that you’ve described would fit perfectly into a faulty vacuum check valve or “sucking jet pump” for the vacuum power booster. The power brakes on your E39 5-series (97-03), as well as many other BMW and MINI models, are operated by a vacuum booster that amplifies the pressure that is applied to the brake master cylinder when you press on the brake pedal. A key exception to this is some of the early 6-cylinder and V12 models up through the early ’90s. These models use a hydraulic fluid boost system.
Typical Sucking Jet Pump:
Typical Check Valve:
The vacuum for the vacuum assisted power brake system comes from the engine’s intake manifold. While the engine is running, the intake manifold is normally under a vacuum (internally) and this vacuum is applied to the brake booster. There are engine operating conditions when the vacuum may be low, such as cold-start, acceleration and stop & go driving. When the vacuum is low, the power brakes may not be adequately boosted through the vacuum booster. To help eliminate this issue, there is a one-way check valve in the hose that connects the engine (intake manifold) to the vacuum booster. Additionally, some models also have a device known as a sucking jet pump. This is nothing more than a vacuum divider, but it does incorporate internal check valves for the vacuum to the brake booster. The check valve (and the valves in the sucking jet pump) allows vacuum to be applied to the booster when the engine is producing higher vacuum, but does not allow the vacuum to be dissipated out of the booster when the engine’s vacuum is low. This keeps the power brake assist available through the conditions when there is not enough engine vacuum to properly operate the vacuum booster.
Without having your car here to diagnose in-person, we can’t be certain exactly what the fault is. However, we would not have suggested the repairs that have already been performed. For a “virtual” diagnosis, we would suggest that you replace the vacuum check valve as your described symptoms certainly can be attributed to a faulty check valve. If the check valve does not cure the issue, the sucking jet pump may be at fault. On some models, the check valve is a disc-like plastic unit in-line in the vacuum hose from the engine to the vacuum booster. Your 530i incorporates the check valve into the 90-degree elbow that connects the vacuum hose to the booster. Models that incorporate the sucking jet pump (such as your 530i) have the unit installed in-line in the vacuum hoses between the intake manifold and the vacuum booster.
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