BRAKE VALVE

[dave lowry]

Earlier today I responded to a question from Jerry Lader about a problem
with the brake lock that I'd mentioned in my "Trip Report". I had intended
my response to go the whole group since I was "fishin'" for comments, but
didn't notice it was off net. So here's Jerry's question and my response
for y'all to check for faulty logic. Have at it!

> >Hi Dave,
> >Did I read that you had trouble releasing your brake valve on your recent
> >trip? I am very interested in installing a valve like you have. Please tell
> >me what you found wrong.
> >Jerry Lader Cad. Powered Edgemonte(CADDYSHACK)
> > hot and humid today in Memphis

Response:
> As I understand it, the way these things work is related to why they can
> also lock up. When the lock is actuated, pressure is already being applied
> to the wheel cylinders by having your foot on the brake pedal. The lock
> simply prevents that pressure from being returned to the master cylinder
> until pressure greater than what is being held back is applied from the
> master cylinder side. When that happens, the pressure is the same on both
> sides for an instant allowing the valve to retreat and fluid to again flow
> in either direction. It locks up, or won't release, when you can't generate
> greater pressure with the pedal.
>
> I believe this happens when the pressure being held back increases due to
> radiated heat along the various brake lines. Both times it occurred, we
> were at elevation and set it after pulling a grade and while stopped on hot
> blacktop. When it happens, you have two choices; wait until the lines cool
> a little (might be1/2 hour), or crawl underneath and open a bleeder valve.
> For us, waiting is the only option.
>
> I'm not sure anything is really wrong. Its just a circumstance that isn't
> present in the lock's normal application.
>
> This is admittedly theory at this point, but until someone offers a more
> plausible explanation, I'm going to assume it to be valid.
>
> Last, here's what I'm planning to do about it. I'll have by mechanic
> install a bleeder valve just after the lock and accessable from under the
> driver's side engine compartment door. My wife can the be taught how to
> release enough pressure to get us going again. Since we carry brake fluid,
> we can check and fill the master cylinder if necessary.
>
> I hope I haven't confused more than clarified, and that if I'm the one
> confused, one of the GMCnet "experts" will straighten me out.
>
> Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA

 

[gene]

There must be a better answer. I think it is very risky to bleed the brake
fluid.

if you were outside the coach there is a chance it could get away from you.

There would be a chance of comprising the rear brakes and loss of all rear
function. You want your brake system sealed and not open except during
controlled conditions like when in service.

What you were looking for earlier this year was a way normally closed by
bass around the brake lock. A second lock is not the answer. A manual
bypass valve around the lock, would let you open the line around the lock
and release the pressure. This would work and would not put anything in
the brake system that could be a threat to the normal operation. We need
to think about this, there must be a better answer.

The part of the problem is the brake is set too hard, but this can happen
so there has to be a fail safe installation.

The lever actuated locks do not have this problem, but the install is not
as easy. The Electric locks, may or may not have this problem, but they
are dependent upon power which creates problems of their own.

I wonder if a Brake Buddy on the output side of the lock would keep the
pressure from going too high??? Will have to ask Duane.

There should be an easy answer here.
gene

>Earlier today I responded to a question from Jerry Lader about a problem
>with the brake lock that I'd mentioned in my "Trip Report". I had intended
>my response to go the whole group since I was "fishin'" for comments, but
>didn't notice it was off net. So here's Jerry's question and my response
>for y'all to check for faulty logic. Have at it!
>
>> >Hi Dave,
>> >Did I read that you had trouble releasing your brake valve on your recent
>> >trip? I am very interested in installing a valve like you have. Please
tell
>> >me what you found wrong.
>> >Jerry Lader Cad. Powered Edgemonte(CADDYSHACK)
>> > hot and humid today in Memphis
>
>Response:
>> As I understand it, the way these things work is related to why they can
>> also lock up. When the lock is actuated, pressure is already being applied
>> to the wheel cylinders by having your foot on the brake pedal. The lock
>> simply prevents that pressure from being returned to the master cylinder
>> until pressure greater than what is being held back is applied from the
>> master cylinder side. When that happens, the pressure is the same on both
>> sides for an instant allowing the valve to retreat and fluid to again flow
>> in either direction. It locks up, or won't release, when you can't generate
>> greater pressure with the pedal.
>>
>> I believe this happens when the pressure being held back increases due to
>> radiated heat along the various brake lines. Both times it occurred, we
>> were at elevation and set it after pulling a grade and while stopped on hot
>> blacktop. When it happens, you have two choices; wait until the lines cool
>> a little (might be1/2 hour), or crawl underneath and open a bleeder valve.
>> For us, waiting is the only option.
>>
>> I'm not sure anything is really wrong. Its just a circumstance that isn't
>> present in the lock's normal application.
>>
>> This is admittedly theory at this point, but until someone offers a more
>> plausible explanation, I'm going to assume it to be valid.
>>
>> Last, here's what I'm planning to do about it. I'll have by mechanic
>> install a bleeder valve just after the lock and accessable from under the
>> driver's side engine compartment door. My wife can the be taught how to
>> release enough pressure to get us going again. Since we carry brake fluid,
>> we can check and fill the master cylinder if necessary.
>>
>> I hope I haven't confused more than clarified, and that if I'm the one
>> confused, one of the GMCnet "experts" will straighten me out.
>>
>> Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA
>
>
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/

 

[gene]

As I think more about this, I think the brake buddy might be an answer.

The brake buddy tries to even out the pressure pulses on the brakes as the
shoes move in and out and the temperature rises. Brake systems have high
pressures but low volume so the brake buddy is a little pressure dash pot
that absorbs pressure pulses to keep even pressure on the brakes. It is
normally connected between the master cylinder and the brakes.

If we put it on the output of the brake lock it should absorb the pressure
increase caused by the heating of the brake fluid....sort of an expensive
fix but we would get the advantage of the brake buddy on the brakes as well
as solve this problem....

gene

>Earlier today I responded to a question from Jerry Lader about a problem
>with the brake lock that I'd mentioned in my "Trip Report". I had intended
>my response to go the whole group since I was "fishin'" for comments, but
>didn't notice it was off net. So here's Jerry's question and my response
>for y'all to check for faulty logic. Have at it!
>
>> >Hi Dave,
>> >Did I read that you had trouble releasing your brake valve on your recent
>> >trip? I am very interested in installing a valve like you have. Please
tell
>> >me what you found wrong.
>> >Jerry Lader Cad. Powered Edgemonte(CADDYSHACK)
>> > hot and humid today in Memphis
>
>Response:
>> As I understand it, the way these things work is related to why they can
>> also lock up. When the lock is actuated, pressure is already being applied
>> to the wheel cylinders by having your foot on the brake pedal. The lock
>> simply prevents that pressure from being returned to the master cylinder
>> until pressure greater than what is being held back is applied from the
>> master cylinder side. When that happens, the pressure is the same on both
>> sides for an instant allowing the valve to retreat and fluid to again flow
>> in either direction. It locks up, or won't release, when you can't generate
>> greater pressure with the pedal.
>>
>> I believe this happens when the pressure being held back increases due to
>> radiated heat along the various brake lines. Both times it occurred, we
>> were at elevation and set it after pulling a grade and while stopped on hot
>> blacktop. When it happens, you have two choices; wait until the lines cool
>> a little (might be1/2 hour), or crawl underneath and open a bleeder valve.
>> For us, waiting is the only option.
>>
>> I'm not sure anything is really wrong. Its just a circumstance that isn't
>> present in the lock's normal application.
>>
>> This is admittedly theory at this point, but until someone offers a more
>> plausible explanation, I'm going to assume it to be valid.
>>
>> Last, here's what I'm planning to do about it. I'll have by mechanic
>> install a bleeder valve just after the lock and accessable from under the
>> driver's side engine compartment door. My wife can the be taught how to
>> release enough pressure to get us going again. Since we carry brake fluid,
>> we can check and fill the master cylinder if necessary.
>>
>> I hope I haven't confused more than clarified, and that if I'm the one
>> confused, one of the GMCnet "experts" will straighten me out.
>>
>> Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA
>
>
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/

 

[dave lowry]

>There must be a better answer. I think it is very risky to bleed the brake
>fluid.
>
>if you were outside the coach there is a chance it could get away from you.
>
>There would be a chance of comprising the rear brakes and loss of all rear
>function. You want your brake system sealed and not open except during
>controlled conditions like when in service.
>
>
>What you were looking for earlier this year was a way normally closed by
>bass around the brake lock. A second lock is not the answer. A manual
>bypass valve around the lock, would let you open the line around the lock
>and release the pressure. This would work and would not put anything in
>the brake system that could be a threat to the normal operation. We need
>to think about this, there must be a better answer.
>
>The part of the problem is the brake is set too hard, but this can happen
>so there has to be a fail safe installation.
>
>The lever actuated locks do not have this problem, but the install is not
>as easy. The Electric locks, may or may not have this problem, but they
>are dependent upon power which creates problems of their own.
>
>I wonder if a Brake Buddy on the output side of the lock would keep the
>pressure from going too high??? Will have to ask Duane.
>
>There should be an easy answer here.
>gene

....and then you wrote

As I think more about this, I think the brake buddy might be an answer.

The brake buddy tries to even out the pressure pulses on the brakes as the
shoes move in and out and the temperature rises. Brake systems have high
pressures but low volume so the brake buddy is a little pressure dash pot
that absorbs pressure pulses to keep even pressure on the brakes. It is
normally connected between the master cylinder and the brakes.

If we put it on the output of the brake lock it should absorb the pressure
increase caused by the heating of the brake fluid....sort of an expensive
fix but we would get the advantage of the brake buddy on the brakes as well
as solve this problem....

gene

I thought about a closed by-pass loop also, but am leaning toward the bleed
valve for two reasons; 1) Simple installation. It should only require a tee
and some brake line to put the valve where it would be fairly accessable,
and 2) It would be used so infrequently and release so little fluid as to
have just a minor effect on fluid level. The valve itself should be no more
risky than the other 6 already in the system.

When the lock is set using just enough pressure to keep the coach from
rolling, it doesn't build enough pressure that it can't be released. The
problem only occurs when the pedal is pushed really hard when setting.
Twice now Dege has forgotten and pushed a little harder than necessary
("just to make sure"). The lock is only intended to be used to prevent the
tranny from being locked in "park" when parking on a slope, and only for
very brief periods. When its necessary to park on a slope for any longer
time, we use wheel chocks, release the brakes to let the coach roll against
the chocks, and then put the tranny in park.

Re: the "Brake Guard", I must not understand its pressure compensating
capabilities of very well. I thought it was designed to attenuate very
brief spikes in pressure, like from warped rotors. Since the lock up
problem results from a gradual build up, it doesn't seem like it would help.

Until we have a way to release a "lock up", we'll have to just be carefull,
huh?

Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA