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Old 08-23-2016, 01:11 PM   #1
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Air brakes, 1991 Monaco

My Monaco doesn't stop like I think it should. It passes every air brake test. In an emergency stop it takes all the effort I can put on the pedal and will not slide a tire. Since the system is the same as a truck and we've all seen them lock it up, I wonder if my Monaco is under braked to prevent this? I've had special higher friction shoes made and all the drums turned. It actually stops better when towing my 6000lb trailer with electric brakes turned all the way up. I don't like having to step on the end of the pedal with max effort. Any help?
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Old 08-23-2016, 02:29 PM   #2
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Does it have self adjusting brakes? In a 91 it may have them or not. If so the self adjusters need attention. If not the manual adjustment is simple to do.
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Old 08-23-2016, 06:48 PM   #3
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Go to the Monaco owners site here and ask the question.

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Old 08-23-2016, 06:58 PM   #4
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3 things come to mind.

Check for slack adjuster movement. 1 inch is good, 3 inch is bad.

What pressure is your air system running. Maybe adjust the governor up to 125 cutout pressure.

I have replaced shoes that had leading and following brake blocks. They had to be put in the right position, according to the side of the axle and S cam position.

The trucks skidding the tires are usually the empty ones.
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Old 08-23-2016, 11:17 PM   #5
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Has manual slack adjusters which are adjusted correctly. Air shutoff is set at 130lbs.
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Old 08-24-2016, 09:55 PM   #6
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Most air brake systems require only 15-20 lbs of air for a basic stop.... 50 lbs of air applied to the spring brakes should lock the shoes... a simple test gauge in the line will confirm and test the applied air... The foot pedal/treadle is designed to add air pressure as needed....

The over all movement of the slack adjuster of very important... 1 - it should be at a 90* angle to the spring brake rod when the shoes contact the drum... thus applying maximum torque to the s-cam... 2 - a spring brake has limited movement... so you need to adjust the slack adjuster so that the shoes almost contact the drum.... AND total movement of the rod in the spring brake is less than 1 1/2".. 3 - spring brakes come in different sizes and all mount to the same bolt pattern... there are 20/20, 24/24, & 30/30's the number provides the area in sq inches in the diaphragm... if your chassis came with one size.. and someone decided to replace them with a different size... say from a 30/30 to a 24/24 than you would experience limited braking... 4 - if the brake pedal/treadle is not passing the correct air pressure to the valve at the axle... the system can't function correctly...

And last... if as you say... it was inspected by a licensed tech who said they were fine.... you need to find a new tech... your complaint is a simple basic complaint... The manufacture of the chassis would have designed the chassis to specific DOT standards... and it MUST by those standards and laws stop within a specified distance at any given speed....

Twinboat is somewhat incorrect on air brake shoes... it is common for shoes to have a primary and secondary friction on the shoe... however the two shoes are interchangeable on the axle.... this is different than on a car or light truck... When the shoes are installed on the backing plate, the friction closest to the s-cam is one formula and the friction closest to the anchor is a different friction... On a car or light truck... the two shoes are connected together and float... thus the leading shoe is one formula and the trailing shoe is a different friction... On an air brake system the anchor is fixed and won't move...

Most truck air systems use compressors that have a 15% duty cycle.. that means that when the engine runs the compressor is designed to run about 15% of the time... it actually runs all the time... but the governor turns on/off the intake valves so it cycles... these compressors have aluminum pistons and aluminum rods... changing the governor and increasing the air pressure does 2 things.. 1 - the duty cycle increases and it can over heat... just like it would if there was a leak... 2 - the added pressure from increasing the system pressure can cause premature system failure (bent rods, broken pistons) from excessive pressure and added heat...
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Old 08-25-2016, 10:23 AM   #7
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Thank-you Jelag

Thank-you for your input. I have been an auto mechanic for 50yrs so I did the work myself. Unfortunately air brakes aren't on cars and good info as to pressures is hard to get. I have put up with the poor braking for over 10 yrs. The shoes are original and I had them rebuilt with a higher friction material. The S cams are in good shape and not loose or binding. I adjust the slack adjusters until lockup and back them off till the shoes just stop touching. Leak tests are well within time and pressure parameters as well as pressure build up times. Nothing I have done has improved braking. I can actually bury the pedal to the stop and have many times in a panic stop (panic because it's so slow to stop). We use the coach every weekend 6mos of the year towing a 6000lb car trailer which as I say stops better with the trailer. Thanks for the pressure numbers, I will pursue that avenue next. Hoping a parts dealer can tell me which size cans came on it originally.
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Old 08-25-2016, 02:32 PM   #8
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I have found with most slack adjusters, tightening up until tight and backing off 1/4 turn, puts you in the 1" brake chamber rod, movement zone.

This is, of course, with the tires chocked, parking brake off and full air tanks.

If your backing the adjuster off more then a half turn, that may be part of your problem.

Once adjusted, having someone step, moderately, on the brake pedal and measuring the travel of the brake chamber rod will confirm the proper adjustment.
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Old 08-25-2016, 03:00 PM   #9
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Air pressure shows 90psi at the trailer connector with peddle buried. Front cans are 16's, can't find number on backs. Considering going to 20's on the front? Likely would need to up rears the same percentage?
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Old 08-25-2016, 03:53 PM   #10
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My understanding is that you should see tank pressure at the brake chambers.

Unless you have a regulator, limiting psi to the trailer connector, I would be looking at why your not getting tank pressure to the brake chambers.

If you have a floor mounted treddle valve, do you have a carpet or mat under your brake pedal ?
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Old 08-26-2016, 06:38 PM   #11
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Relay Valves
Relay valves are primarily used on vehicles to apply
and release rear axle(s) service or parking brakes.
When the driver applies the brakes, air travels through
the delivery (in this case signal) line to the relay valve
and moves an internal piston down. This closes the
exhaust and opens the delivery of air to the brakes.
The primary benefits of using a relay valve is that the
high capacity of air needed for braking is delivered
directly and does not have to travel up to the brake
valve and then all the way to the brakes. The brake
force is adjustable and when released the relay valve
exhausts to atmosphere
. Relay valves are generally
mounted close to the chambers they serve and are
available in both remote and reservoir mount designs.
The inlet/exhaust valve cartridge can be replaced
without line removal.

HTH
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Old 08-27-2016, 09:35 AM   #12
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Definitely something wrong. Higher friction material is just hiding the problem. Half peddle should put you through the window. Air brake systems are not without their own unique problems. An air gauge is your friend but it takes time to find the issue. Relay valves are common failures but it could be your treadle valve (foot peddle) they are know to fail. Kinked air line somewhere will allow some signal to get through but not enough in a timely fashion. Changing to bigger pots is a recipe for disaster leading to the real possibility of causing an accident by more front end braking and causing a skid. Front and back are designed to work together usually 20 in the front and 26 or 30's on the rear. I also don't mean to criticize but relining your own shoes might make it worse as the shoes don't conform to drum diameter. In the years past (trailer mechanic) sometime we would find a real bad trailer and on those specifically (tractor too) would turn the drums AND shoes on the axle to make sure of 100% mating surface. Relay valves would be second choice, the actual brake valve would be my first check (I happen to prefer R-6) it is a dated model but most trouble free style (unless you have anti-lock). Relay valves, second but also check out the treadle valve. Here is a GENERIC air brake system. It is non specific but is how they work and the general layout of the system.


NO IT'S NOT NORMAL.

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Old 08-27-2016, 01:04 PM   #13
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I have to add some more... I'm guessing that you will NEVER see tank pressure at the spring brake... remember the brake pedal sends a signal only down a small line to the valve at each axle.... see what sjholt has written.... this signal air than throttles the relay valves at each axle... each relay valve has a what is called crack pressure.... that crack pressure is the air pressure from the throttle valve that makes it first open... this is very important on multi trailer and multi axle applications...

So the brake pedal cracks open the relay valve and the relay valve supplies a large volume of air... at a varied pressure... you want more brake, you press the brake pedal harder and it signals the relay valve and it opens farther and supplies more air volume and a higher pressure....

If you have NO LEAKS than you have one of about 4 problems... 1 - the line from the tank to the relay valve, or the line to the spring brakes is plugged.... 2 - the brake pedal is not sending a strong enough signal to the relay valve... 3 - the relay valve is not opening as it should and sending air volume/pressure to the spring brakes.. 4 - you might be exceeding the travel limit of the spring brake because of improper installation, pull the clevis from the slack adjuster and check the travel...

I WOULD NOT, that is WOULD NOT ever change a spring brake to a larger size than what has been supplied by the chassis manufacture.... I would diagnose the problem and repair it....

I'm sure you'll notice that the front spring brake is a single chamber... the rear chambers are a dual chamber.. the second chamber is the parking/emergency brake.... DO NOT, DO NOT ever try and disassemble a dual chamber spring brake... the parking brake spring is a lot like the front coil spring on a car... something that can cause severe damage if it is turned loose without being caged....

Any certified air brake tech should be able to diagnose this with simple gauges and repair the problem... remember I said certified... to work on truck air brake systems technicians are required to complete a training program, and carry a certification card.... I can't tell you how many of these classes I've either sat in or sponsored...

Usually NAPA, or any of the truck dealers offer a class for their customers.... they are usually free... call your local truck dealer, or NAPA (must be a HD supplier) and ask ""when is your next HD Brake Certification Class for your customers""... Because we re-manufactured these shoes we put a lot of these classes on for our customers....

NOW your RV is a pre 94 which means it has manual slack adjusters... and you've read above about adjusting these.... also on all my vehicles we checked and adjusted up the auto slack adjusters every 90 days... I liked to know that these slacks are working correctly....

OF COURSE there can be other issues... but what I've talked about are the MOST COMMON issues that mechanics have issues with...
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Old 08-27-2016, 02:24 PM   #14
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Quote:
Originally Posted by jelag View Post
I have to add some more... I'm guessing that you will NEVER see tank pressure at the spring brake... remember the brake pedal sends a signal only down a small line to the valve at each axle.... see what sjholt has written.... this signal air than throttles the relay valves at each axle... each relay valve has a what is called crack pressure.... that crack pressure is the air pressure from the throttle valve that makes it first open... this is very important on multi trailer and multi axle applications...

So the brake pedal cracks open the relay valve and the relay valve supplies a large volume of air... at a varied pressure... you want more brake, you press the brake pedal harder and it signals the relay valve and it opens farther and supplies more air volume and a higher pressure....

If you have NO LEAKS than you have one of about 4 problems... 1 - the line from the tank to the relay valve, or the line to the spring brakes is plugged.... 2 - the brake pedal is not sending a strong enough signal to the relay valve... 3 - the relay valve is not opening as it should and sending air volume/pressure to the spring brakes.. 4 - you might be exceeding the travel limit of the spring brake because of improper installation, pull the clevis from the slack adjuster and check the travel...

I WOULD NOT, that is WOULD NOT ever change a spring brake to a larger size than what has been supplied by the chassis manufacture.... I would diagnose the problem and repair it....

I'm sure you'll notice that the front spring brake is a single chamber... the rear chambers are a dual chamber.. the second chamber is the parking/emergency brake.... DO NOT, DO NOT ever try and disassemble a dual chamber spring brake... the parking brake spring is a lot like the front coil spring on a car... something that can cause severe damage if it is turned loose without being caged....

Any certified air brake tech should be able to diagnose this with simple gauges and repair the problem... remember I said certified... to work on truck air brake systems technicians are required to complete a training program, and carry a certification card.... I can't tell you how many of these classes I've either sat in or sponsored...

Usually NAPA, or any of the truck dealers offer a class for their customers.... they are usually free... call your local truck dealer, or NAPA (must be a HD supplier) and ask ""when is your next HD Brake Certification Class for your customers""... Because we re-manufactured these shoes we put a lot of these classes on for our customers....

NOW your RV is a pre 94 which means it has manual slack adjusters... and you've read above about adjusting these.... also on all my vehicles we checked and adjusted up the auto slack adjusters every 90 days... I liked to know that these slacks are working correctly....

OF COURSE there can be other issues... but what I've talked about are the MOST COMMON issues that mechanics have issues with...
There is some confusing information here.

In the air brake industry, the "Spring Brake" is a dual chamber, consisting of a service and emergency/parking brake section.

It has 2 air lines connected to 2 independent ports, marked service and emergency.

The service port supplies air to apply the brakes. ( pushs the rod out )

The emergency port supplies air to the spring side of the chamber to hold off the emergency/parking brake. ( pulls the rod in )

Spring brakes are not used in the front axles.

The stuff about the relay valve is valid.

You can safely cage a spring brake dual chamber. Once caged, the service diaphram can be replaced.

The spring side on today's chamber is crimped and unservicable. Older ones were not, and if caged properly, could be opened.
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