| Hydraulic Parking Brake Safety [message #220981] |
Tue, 03 September 2013 04:57  |
djeffers
Messages: 219
Registered: June 2004
Karma: 2
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You will typically generate no more than 1,000 psi max in your brake lines when you make a hard stop with your vehicle.
The maximum pressure you might ever reach is about 1,500 to 1,700 psi.
At these pressures, if you are using a hydraulic line lock or a mechanical mechanism to hold the brake line pressure as a parking brake:
YOU NEED ONLY TO LOSE LESS THAN 1/2 TEASPOON OF BRAKE FLUID FOR THE BRAKE LINE PRESSURE TO DROP TO ZERO.
This is due to the very low compressibility of brake fluid. Doesn't compress much, doesn't have to loose much. No pressure, no brakes.
Our GMC coaches use a dual brake system, the two front brakes and the four rear brakes. If the rears go, you are depending on the two front brakes to hold the coach.
The coach can roll away if you are depending on holding brake line pressure to use for a parking brake and 1/2 teaspoon of brake fluid leaks out.
Possible leak modes are past the brake piston seals, out a failed steel brake line due to rust, out one of the many brake line connections or if a flexible brake hose fails. The coach can roll away.
You might not notice such a small leak point when you use the service brakes as you seldom generate high brake pressures and the master cylinder reservoir will replenish the loss with a little more travel of the brake pedal.
The leak may be at a point where it doesn't drop on the pavement, but just drips on the frame rail or another out of sight place.
The commercial, purpose made brake line lock mechanisms recommend resetting the brake line pressure with the brake pedal once an hour. How long will you leave yours parked, unattended? They also recommend using them only in addition to the mechanical parking brake.
The last time I looked (a couple of years ago) SAE J1401 required a burst pressure strength of 4,000 to 5,000 psi for new flexible brake hoses.
4,000 to 5,000 psi is far above what you can generate with the brake system in your vehicle. A failure of a flexible brake hose under pressure is the result of a defective flexible brake hose. Using new DOT approved hoses will guarantee that burst strength.
A corroded steel line can function well and then fail at a high pressure stop. A corroded steel line can also just reach the failure point in normal light use one day, and you have lost those brakes.
If you are using brake line pressure for a parking brake, you are stressing that coming failure point for a much longer cumulative time and pressure than when you are just using the service brakes occasionally and moderately.
If you ever use holding brake line pressure by some method for a parking brake, blocking the wheels with effective chocks is a good upgrade. Don't be embarrassed, don't be shy. Use big effective chocks if you don't have an effective mechanical parking brake.
The bottom line is, under some conditions you must have an effective mechanical parking brake mechanism to have a safe vehicle to park. Some states require it by law, others make no such requirement.
Your coach had to meet Federal Safety Standards with respect to the parking brake when new, it can be maintained to that same standard.
Don't risk killing or injuring someone by not having effective parking brakes. Only a few vehicles using brake line pressure for parking will fail, but fail some will and a few of those will injure or kill someone.
Any brake line pressure parking brake mechanism should be used in addition to a mechanical parking brake, not in place on one.
Stop the coach, engage that good mechanical parking brake, put it in Park and then engage your brake line pressure parking brake. If you don't have a good mechanical parking brake, be especially careful where you park, chock the wheels and consider what getting good mechanical parking brakes will do for the safety of your coach.
Soap box out.
Don & Susan Jeffers
78 Eleganza II
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| Re: Hydraulic Parking Brake Safety [message #221002 is a reply to message #220981] |
Tue, 03 September 2013 09:23  |
habbyguy
Messages: 896
Registered: May 2012
Location: Mesa, AZ
Karma: 3
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Senior Member |
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| djeffers wrote on Tue, 03 September 2013 02:57 |
You might not notice such a small leak point when you use the service brakes as you seldom generate high brake pressures and the master cylinder reservoir will replenish the loss with a little more travel of the brake pedal.
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I agree with your post (including the fact that the hydraulic parking brake shouldn't be your only parking brake, which is a recurring theme through all these posts by pretty much everyone).
I just wanted to add that your pedal travel shouldn't change if your brake system does have a little leak, at least as long as air doesn't get into the system... your master cylinder reservoir will simply replace the missing fluid until it's empty (in which case it starts replacing missing fluid with air, with predictable results).
I guess you could extrapolate that to mean that if someone finds that they need to add the occasional top-off to their brake fluid under normal use, a hydraulic parking brake is a questionable proposition... at least for more than VERY temporary use (like backing up the regular parking brake while placing chocks under the wheels). Those who don't ever have to add fluid may get more use out of a hydraulic "parking" brake (again, on a very temporary basis).
Mark Hickey
Mesa, AZ
1978 Royale Center Kitchen
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