Home » Public Forums » GMCnet » Q. for Wally
| Q. for Wally [message #267505] |
Sat, 13 December 2014 08:55  |
g.winger
Messages: 792
Registered: February 2008
Location: Warrenton,Missouri
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Why,,,, are you testing brake pressure? I think I,ve seen a bunch of work others have done. Not being critical,,,, is someting wrong.? Did some of your pressure run off???
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| Re: Q. for Wally [message #267546 is a reply to message #267505] |
Sun, 14 December 2014 15:24  |
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wally
Messages: 643
Registered: August 2004
Location: Omaha Nebraska
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g.winger wrote on Sat, 13 December 2014 08:55Why,,,, are you testing brake pressure? I think I,ve seen a bunch of work others have done. Not being critical,,,, is someting wrong.? Did some of your pressure run off???
Paul, I just want to know what the pressure is. Have not done it yet mostly because I have had no one to look at the gages while I'm on the pedal. I did see Bob Stone had a picture of 2,000 psi with his hydroboost conversion. Testing at the wheels will show what the proportional valve is really doing.
Wally Anderson
Omaha NE
75 Glenbrook
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| Re: [GMCnet] Q. for Wally [message #267547 is a reply to message #267546] |
Sun, 14 December 2014 17:08 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Wally,
Please don't take this negatively but as I understand the Combination Valve in a GMC is not really a proportioning valve. Well at
least as I understand the function of a proportioning valve. To me a proportioning valve proportions the front / rear braking
pressure constantly.
Below you will find a paragraph copied out of the Maintenance Manual that describes one of the functions of the Combination Valve.
METERING VALVE FUNCTION (Spring and poppet on left side of Combination Valve)
The metering section of the combination valve operates to 'hold off' hydraulic flow (pressure) until about 130 psi has been built up
in the system before applying the front brakes. The pressure then blends to a full line pressure at approximately 400-600 psi line
pressure. There is no flow restriction when the brakes are released.
Here is a link to a cross section view of the Combination Valve and a description out of the Maintenance Manual that explains what
it does:
http://www.gmcmhphotos.com/photos/aa-miscellaneous-photos/p37696-combination-valve-cross-section.html
Note that fluid enters the combination valve from both pistons in the Master Cylinder, fluid goes directly to the rear brake wheel
cylinders without restriction. Fluid to the front brakes has to overcome the spring pressure of the Metering Valve (about 130 psi)
before it can go to the front brake calipers.
As I understand it the 130 psi means the rear brake shoes are already in contact with the drums before the front pads come in
contact with the disks. This was done to keep the GMC from trading ends when braking heavily especially on wet / slippery surfaces.
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
-----Original Message-----
From: Wally Anderson
Paul, I just want to know what the pressure is. Have not done it yet mostly because I have had no one to look at the gages while I'm
on the pedal. I did see Bob Stone had a picture of 2,000 psi with his hydroboost conversion. Testing at the wheels will show what
the proportional valve is really doing.
--
Wally
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: Q. for Wally [message #267550 is a reply to message #267505] |
Sun, 14 December 2014 19:23 |
mild bill
Messages: 98
Registered: November 2014
Location: Edmonton, Alberta, Canada
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If you lock up the rear brakes without front brakes you will go into a spin, thats why ebrake turns work so cool.. The front brakes do most of the braking, thats why they are the biggest and most resistant to fade. The valves In a front disk rear drum situation delay application of the front disks to allow for brake shoe spring tension and larger shoe to drum clearance and some will also reduce pressure to the rear drums to reduce chance of lock up. The third function is a pressure balance switch between front and rear brake fluid passages and should you lose 1/2 the system an indicator light turns on.
If you have a 4 or 6 wheel disk system or for that matter felt like fine tuning your system all linds of aftermarket valves are available and perhaps not necessary. And of course testing, adjusting and liability would fall on your shoulders.
I guess the best system we have out there is wheel speed sensors on all wheels and a computer regulating pressure.
Lacking that depending on braking equipment changes and suspension changes, it is likely the fronts will still need the most pressure, then the middle wheels and least for the rears. This is evident by the aftermarket design where larger shoes and or wheel cylinders or disks are installed on the middle axle and rear often left stock. just my reasoning due to my education and not practical experience on a GMC motorhome.
[Updated on: Sun, 14 December 2014 19:56] Report message to a moderator |
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| Re: Q. for Wally [message #267552 is a reply to message #267505] |
Sun, 14 December 2014 21:42 |
mild bill
Messages: 98
Registered: November 2014
Location: Edmonton, Alberta, Canada
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All I'm mentioning is rear bogies is the biggest problem with locking up from what I have read (my only experience with lockup was on icy Hwy 2 back to edmonton with brakes that had not been looked at for 14 years and for all I know they all locked up) because of how the suspension works and unloading of the tires so that's where the least pressure or the least amount of brake should be, that's why reaction arm have come out to reduce try to reduce that problem.
I don't know if they work, I bought everything from Manny, one ton, disks, reaction arms and still sitting in boxes.
Picked up a hydra boost and master from a unit where the hydra boost was dripping a bit so it was changed out and thrown in the scrap metal bin so put a kit into it and have plenty of pressure, like the hydra boost I put in my SBC Chevy. They changed out two hydraulic accumulators on a push tractor for some reason for hydra boost so I checked them with 2800 psi nitrogen both directions and no leaks, huge 2" X 28". 2" wheel cyls on 4 about 20" X 10" drums and good for 15 applications, should be good.
All I am thinking is even with the new disks, the only place I can see for a pressure reduction is to the rear bogies, lock up and flat spotting the rear tires isn't something I want. I don't really like computers taking over everything for me. If I could set it up so all 6 tires locked up at the same time that would be just fine, back off on the pedal. If the rears are burning rubber and the others are not I'm nowhere near getting optimum braking,optimum would be just before lockup.
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| Re: [GMCnet] Q. for Wally [message #267553 is a reply to message #267552] |
Sun, 14 December 2014 22:41 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Bill,
Go ahead and install the Manny Brakes and 1-Ton. That will eliminate the
rear wheel lockup and give you close to ideal braking. If you can install
the Hydroboost, do that too. You will not need a proportioning valve for
the rear wheels.
I've got all of it except the Hydroboost, which will be too much trouble to
accommodate with my custom dash.
Before installing the reaction arms, I had not had rear calipers installed
for many years because of the incessant lockups. Now I can make a "panic"
stop without worries -- and I use the same size calipers center and rear.
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI & EBL,
Manny Brakes & 1-Ton, etc., etc.
www.gmcwipersetc.com
On Sun, Dec 14, 2014 at 10:42 PM, Bill Dolinsky wrote:
>
> ...
>
> I don't know if they work, I bought everything from Manny, one ton, disks,
> reaction arms and still sitting in boxes.
> Picked up a hydra boost and master from a unit where the hydra boost was
> dripping a bit so it was changed out and thrown in the scrap metal bin so
> put
> a kit into it and have plenty of pressure, like the hydra boost I put in
> my SBC Chevy. They changed out two hydraulic accumulators on a push tractor
> for some reason for hydra boost so I checked them with 2800 psi nitrogen
> both directions and no leaks, huge 2" X 28". 2" wheel cyls on 4 about 20" X
> 10" drums and good for 15 applications, should be good.
>
> All I am thinking is even with the new disks, the only place I can see for
> a pressure reduction is to the rear bogies, lock up and flat spotting the
> rear tires isn't something I want.
...
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] Q. for Wally [message #267558 is a reply to message #267550] |
Mon, 15 December 2014 04:12 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Bill,
I agree with you if you lock up the wheels in a 4 wheeled car, however, I wonder if that would happen in a GMC as it has a much
longer wheelbase.
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
-----Original Message-----
From: Bill Dolinsky
If you lock up the rear brakes without front brakes you will go into a spin, thats why ebrake turns work so cool.. The front brakes
do most of the braking, thats why they are the biggest and most resistant to fade. The valves In a front disk rear drum situation
delay application of the front disks to allow for brake shoe spring tension and some will also reduce pressure to the rear drums to
reduce chance of lock up. The third function is a pressure balance switch between front and rear brake fluid passages and should you
lose 1/2 the system an indicator light turns on.
If you have a 4 or 6 wheel disk system or for that matter felt like fine tuning your system all linds of aftermarket valves are
available and perhaps not necessary. And of course testing, adjusting and liability would fall on your shoulders.
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: Q. for Wally [message #267562 is a reply to message #267505] |
Mon, 15 December 2014 05:29 |
g.winger
Messages: 792
Registered: February 2008
Location: Warrenton,Missouri
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Wally,,, go pro!!! Should fix the second person problem,,,,,
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| Re: [GMCnet] Q. for Wally [message #267621 is a reply to message #267547] |
Mon, 15 December 2014 18:35 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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""I agree with you if you lock up the wheels in a 4 wheeled car, however, I wonder if that would happen in a GMC as it has a much
longer wheelbase.
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426""
You never want to lock up the rear wheels before the fronts on any vehicle regardless of wheelbase. The rear wheels provide the lateral stability which acts as a reaction point for the front wheels and hence steering. If the rear wheels are not "planted" you can't steer a vehicle because there is nothing to react against. If rear wheels are locked, you lose most of your lateral co-efficient of friction. The reason the fronts are held off momentarily is to allow the rear brake shoes to move outward. It takes a minimum of 70psi for the shoes to overcome the pull back springs and start to move and contact the drums. By design, disk brakes are self adjusting and have little if any clearance between the pads and rotors.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: Q. for Wally [message #267622 is a reply to message #267505] |
Mon, 15 December 2014 18:54 |
mild bill
Messages: 98
Registered: November 2014
Location: Edmonton, Alberta, Canada
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Just want to add that a master cylinder designed for rear drum brakes will have a residual valve that keeps somewhere in the area of 4 to 7 psi in the line to the drum brakes. It is located underneath the brass seat in the output port of the master cylinder and can be removed easily. It's just a little rubber valve and the method I use to remove the seat is to use a tap just larger than the port hole and then screw a machine screw of the same thread size and pry the seat out. Some guys use sheet metal screws however I think a tap does a nicer job. Pull the valve out with a pick or tweezers and after cleaning any cuttings out tap the brass seat back in till you can finish reseating with the brake line, tube nut.
The residual pressure acting on disk brake calliper will result in poorer gas mileage, quicker brake wear and unneeded heat being generated.
We have a lot of units at work that have 2 gmc brake callipers on each rear rotor and it's pretty evident when someone replaces a master cylinder and forgets or didn't know about the residual valve.
[Updated on: Mon, 15 December 2014 18:56] Report message to a moderator |
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| Re: [GMCnet] Q. for Wally [message #267637 is a reply to message #267621] |
Mon, 15 December 2014 22:00 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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I understand the hazard of having rear wheels lock up. But it's my belief
that in the case of the GMC (unless equipped with reaction arms), the rear
wheels will in every case lock up long before the middle ones. So those
center ones will still enable directional control.
I'm curious whether you agree with that opinion, Bob.
Ken H.
On Mon, Dec 15, 2014 at 7:35 PM, Bob de Kruyff wrote:
>
> ...
>
You never want to lock up the rear wheels before the fronts on any vehicle
> regardless of wheelbase. The rear wheels provide the lateral stability
> which acts as a reaction point for the front wheels and hence steering. If
> the rear wheels are not "planted" you can't steer a vehicle because there
> is nothing to react against. If rear wheels are locked, you lose most of
> your lateral co-efficient of friction. The reason the fronts are held off
> momentarily is to allow the rear brake shoes to move outward. It takes a
> minimum of 70psi for the shoes to overcome the pull back springs and start
> to
> move and contact the drums. By design, disk brakes are self adjusting and
> have little if any clearance between the pads and rotors.
> --
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: Q. for Wally [message #267653 is a reply to message #267622] |
Tue, 16 December 2014 08:59 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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I little first hand experience here....
I can lock all six wheels. (We have a 23' that scales at 9460#.)
Shortly after I got the JimK/TomP Reaction Arm (aka Floating Brake) set all tuned in, we took to coach to an open industrial parking lot and played some games. This was several year old asphalt. The tires were decent Transforce.
We could get to 30mph pretty easily in the available room. Either of us could lock all the rear wheels on a hard stop. On a lesser stop we could look 2 out of 4, and those were usually the intermediate, but one time or another, each would lock. When all the rears did lock, there was some loss of line that the driver could easily control. In these conditions the skid only lasted about 8~10ft. The coach never felt like it wanted to switch ends like an empty pickup.
I did two full panic stops from 45mph (this took starting the run out on the road and making the corner kind of fast), then, I could and did skid all six wheels. The situation would have been scary if there had been anything to hit. Control was not good. (Wow, what a surprise?) The stop was amazing. I think my T-coupe or the ALCR each with 4-wheel anti-lock might have done better, but the seat of the pants accelerometer sure put a high value on it. I actually did have my droid on the floor reading out the stop and I wrote the numbers down, but now cannot locate the file.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: [GMCnet] Q. for Wally [message #267662 is a reply to message #267637] |
Tue, 16 December 2014 11:24 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""I understand the hazard of having rear wheels lock up. But it's my belief
that in the case of the GMC (unless equipped with reaction arms), the rear
wheels will in every case lock up long before the middle ones. So those
center ones will still enable directional control.
I'm curious whether you agree with that opinion, Bob.
Ken H.
""
I can't answer that for sure but I do think the leading arm wheels will help if they don't lock. I've only locked my wheels once in 20 years and I hit the pedal so hard that all wheels locked. I wasn't sure whether my fronts could lock but the adrenalin was so high I must have hit the pedal hard!
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] Q. for Wally [message #267663 is a reply to message #267662] |
Tue, 16 December 2014 11:40 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Maximum deceleration effect is always achieved at the point where the tires
are still rotating. When they slide on the traction surface, loss of
steering control becomes a huge factor as well as longer stopping
distances. Applying enough pedal pressure to JUST prevent lockup is the
idea. Hard to remember when you are rapidly approaching a van full of nuns.
That is how ABS systems work. Trained racing drivers with educated feet
can achieve nearly the same results as ABS systems, but for the rest of us,
we have a tendency to "stomp the pedal" during panic stops. GMC coaches
with reaction arm brakes are a large improvement over the stock system,
particularly when equipped with all disc brakes.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Dec 16, 2014 9:24 AM, "Bob de Kruyff" wrote:
> ""I understand the hazard of having rear wheels lock up. But it's my belief
> that in the case of the GMC (unless equipped with reaction arms), the rear
> wheels will in every case lock up long before the middle ones. So those
> center ones will still enable directional control.
>
> I'm curious whether you agree with that opinion, Bob.
>
> Ken H.
>
> ""
>
> I can't answer that for sure but I do think the leading arm wheels will
> help if they don't lock. I've only locked my wheels once in 20 years and I
> hit
> the pedal so hard that all wheels locked. I wasn't sure whether my fronts
> could lock but the adrenalin was so high I must have hit the pedal hard!
> --
> Bob de Kruyff
> 78 Eleganza
> Chandler, AZ
> _______________________________________________
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| Re: [GMCnet] Q. for Wally [message #267664 is a reply to message #267653] |
Tue, 16 December 2014 16:33 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Matt,
This information has answered my question:
"The coach never felt like it wanted to switch ends like an empty pickup.
The wheelbase of a 23' is shorter than a 26' so using my theory it would have a greater tendency to do that.
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
-----Original Message-----
From: Gmclist [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Matt Colie
Sent: Wednesday, December 17, 2014 2:00 AM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Q. for Wally
I little first hand experience here....
I can lock all six wheels. (We have a 23' that scales at 9460#.) Shortly after I got the JimK/TomP Reaction Arm (aka Floating
Brake) set all tuned in, we took to coach to an open industrial parking lot and played some games. This was several year old
asphalt. The tires were decent Transforce.
We could get to 30mph pretty easily in the available room. Either of us could lock all the rear wheels on a hard stop. On a lesser
stop we could look 2 out of 4, and those were usually the intermediate, but one time or another, each would lock. When all the
rears did lock, there was some loss of line that the driver could easily control. In these conditions the skid only lasted about
8~10ft. The coach never felt like it wanted to switch ends like an empty pickup.
I did two full panic stops from 45mph (this took starting the run out on the road and making the corner kind of fast), then, I could
and did skid all six wheels. The situation would have been scary if there had been anything to hit. Control was not good. (Wow,
what a surprise?) The stop was amazing. I think my T-coupe or the ALCR each with 4-wheel anti-lock might have done better, but the
seat of the pants accelerometer sure put a high value on it. I actually did have my droid on the floor reading out the stop and I
wrote the numbers down, but now cannot locate the file.
Matt
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GMCnet mailing list
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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