|View single post by Mark Rosenbaum|
|Posted: 09-18-2005 04:19 pm||
|The symptoms are characteristic of a non-sealing tipping valve inside the master cylinder. Other possibilities in addition to the tipping valve problem include (a) air in the system, as Brett noted, and (b) an insufficient gap between the master cylinder's rear piston rod and the push rod on the front of the brake booster.
The tipping valve, which allows fluid from the reservoir to enter the business part of the cylinder, is normally forced open by the forward lip of the rear piston rod. When the brake pedal is pressed, the piston rod moves forward and releases the valve, which is then supposed to seal. This allows hydraulic pressure for the front brakes to develop. Simultaneously, this developed pressure is applied to the front piston rod, which moves forward to create pressure to operate the rear brakes.
But if the tipping valve fails to seal, pressure for the front brakes never develops, and the forward piston rod does not move until the rear piston rod comes into mechanical contact with it. Only then will the rear brake circuit develop hydraulic pressure.
The condition can be verified by removal of the brake fluid reservoir cap and examination of the reservoir contents while an assistant operates the brake pedal. If the tipping valve is not sealing, you will see a very pronounced backflow of fluid, into the reservoir from the master cylinder. (A very small amount of backflow is normal.)
The problem is inherent in the basic design of the dual-circuit master cylinder. A barely visible piece of debris in the brake fluid, if in the right spot, can be sufficient to keep the tipping valve from sealing. The typical British master cylinder seems more sensitive to such debris than the typical US design, but both can suffer from it. Replacing the master cylinder usually (not always) eliminates the problem, not because the earlier part was defective, but because the debris-containing brake fluid gets replaced with cleaner stuff.
Frequently, the problem is intermittent, with the brakes generally operating normally but failing on occasion. This is because a piece of debris causes the tipping valve to leak, but gets flushed away as the driver repeatedly pumps the brake pedal, and the brakes then operate normally again.
The only actual cure is to use an older style single-circuit master cylinder, or, preferably, one for each brake circuit. It will be recalled that race cars generally arrange their brakes in this manner. Fortunately, those of us who are not driving race cars can deal with the problem by (a) being aware that the brakes may fail at any time without warning, and driving appropriately, (b) ensuring that the brake fluid is always fresh and clean, and (c) replacing it every two years (or annually, in humid environments). Any dust or debris entering the brake fluid from an environmental source can cause the problem. A non-silicone brake fluid will almost instantly absorb considerable moisture directly from the air, and this moisture promotes rust, which is another source of trouble-causing particles.
Attachment: Master cyl cross section.jpg (Downloaded 31 times)
Last edited on 09-18-2005 08:25 pm by Mark Rosenbaum