Braking on a Steep Grade

[bazgor]

I went down a steep grade with my 93 bounder, 28', P30 chassis.

When I reached the bottom, the brakes were hot. I am also towing a small suv. I Drove another 10 miles and pulled over at the next town for a break.

One half hour later, I went to leave. I put my foot on the brake to start it, and when I started it the brake pedal went all the way to the floor. I pumped the brakes a few times and still nothing. I waited another few moments and pumped the brakes again, and then they came back to normal. Does anyone know why it did that. Is there any solution for braking going down a steep incline, like an engine brake.

Thanks Gord.

[wolfe10]

Gord,

I am sure you will get some good firsthand information on your specific brake failure question.

A couple of general comments:

Be sure that you change you brake fluid every 2-3 years. As it absorbs water (as it was designed to do to protect the iron parts of you brake system) the boiling point of brake fluid decreases from close to 500 degrees F to 286 degrees F! Temperatures in your calipers under extended hard braking can exceed 286.

The proper use of your service brakes while descending a grade is to NOT USE THEM. They are not there to help you maintain a safe speed of descent. They are ONLY to be used to slow you down enough to "grab a lower gear." Though your coach weighs many times what your car weights, brake surface area (dictated by size of wheels) is only slightly larger than on your car. So a very different driving technique is needed.

The correct gear to descend a grade is the gear that holds your speed in equilibrium. That could mean 1st or 2nd gear, even if the road is dead straight for 10 miles. If you find that your speed is increasing, firmly apply the brakes enough to slow down enough to "grab a lower gear." Physics dictates that your equilibrium speed is slower than an empty 18 wheeler and faster than a loaded one.

We assume that your toad has brakes as well. Your chassis brakes were not designed to stop the weight of the coach AND the toad.

If you drive properly, you will not wear out coach brakes -- we have 158,000 miles on original brakes and have driven a LOT of serious mountain roads.

Brett

[bazgor]

Thank you for the info. Do you know where the master cylinder is located on a p30 chassis

[hermanmullins]

Gord,

You should find your master cylinder under your hood just to your right while facing the coach. Gord, I'm not trying to be smart with you but if you are having a hard time finding the master cylinder, might I suggest that you take your coach to good shop and let them flush and refill your brake fluid. (watch what they do and you can learn how to do it the next time yourself) At the same time you might have them look at your brake shoes or pads.

Good Luck and Happy RVing.

[desertdeals69]

Some years ago I spoke to a fellow that had a Pace Arrow 36 and he said at the top of a steep hill he would slow down enough to put in into low gear and proceed down the hill at 10-15 mph otherwise he would overheat his brakes. That is one of the advantages of a diesel is the exhaust brake. I have gone down more than 9% downgrades and never had to use the brakes running 35-45 mph.

[johnklopp]

A little more explanation is in order for you to understand why the brake pedal went to the floor. The moisture in the fluid actually boils and created a vapor pocket in the hot brake cylinder. Once the brake cylinder cools down the vapor condenses and the brakes return to normal. Changing fluid is a good precaution and should/must be done periodically to eliminate moisture.

Another point is the DOT rating of the brake fluid. The high the DOT rating the higher the boiling point. DOT 2 (cheap) brake fluid boils at hundreds of degrees less than DOT 5 brake fluid (expensive). The next time you’re at the auto parts store take a look at the brake fluid. The boiling point will be labeled on each can.

[wolfe10]

Since his "brake pedal to the floor" occurred AFTER stopping for 30 minutes, I doubt boiling brake fluid was the cause.

I am not that familiar with the hydroboost brake system that I suspect he has, that is why I did not speculate on the cause of the pedal goes to the floor.

Brett

[hermanmullins]

Hydroboost is one of the worst thing GM ever put on a Truck Chassis. I drove a 93 Chevrolet 1 ton with Hydroboost and the heavier the load the less brakes you had. If Gord has Hydroboost likes his coach other then the Brakes he might check into seeing if he can convert to a Vacuum Booster system. I don't know if it can be done but I would sure check it out.

Gord I don't know how you approch a steep down hill grade, but down shifting to a lower gear will help.

[desertdeals69]

Using a lower gear is dependent on the torque converter, how good it locks up or if at all.

[robertbeers]

A lot of good info in this thread. It will take more than one purge of your brake fluid to get all the water and burnt fluid out of the system so get a quality shop to do the work for you. New fluid needs to be a higher temp and synthetic brand. Your braking goal is to never touch the brakes. That is an impossible task but a good goal. On hills you go down hills in the same gear you would use going up that same hill. If you ever again encounter the same problem, you are using the brakes too much and you need fluid replaced again because it is burnt. As stated above our P30 series brakes are marginal at best so they need to be kept in top condition.

[robertbeers]

Also; Assuming your coach is a class A, the master cylinder is just to the right of your feet under the floor. On My Trek it is accessible from the engine cover inside the unit.

[Sundancer268]

DOT 5 Brake Fluid is a silicone fluid and IS NOT compatable with any other brake fluid. There is a new DOT 5? brake fluid which is compatable but you will probably have a hard time finding it. If you want to try and find the new DOT 5 make sure it is compatable with the older DOT standards and not the silicone type. My 1982 Harley used the DOT 5 and some specially built race vehicles used it also. Maybe some exotic foreign cars will also use it. Just replace it with DOT 4 or DOT 3 and change it every 2-3 years.

[wolfe10]

I agree with Sundancer. Do not change to a silicone-based fluid. Ford has one of the best brake fluids compatible with your system with a new boiling point right at 500 degrees F.

Brett

[OkieDave]

The brake reservoir on the P32 chassis and probably on the P30 can be reached from the left front wheel well. I know, it does not make sense but that is where it is. Look just inside and above the frame.

As far as using the DOT 5. DON'T.

The is from the Workhorse Chassis manual that covers P30-P32.

Do not add DOT 5 brake fluid to the master cylinder reservoir. DOT 5 fluid is silicon base whereas the correct DOT 3 fluid is Glycol based. The two will not mix and the DOT 5 fluid can cause major damage to the anti-lock brake module and other brake components

[hermanmullins]

It is always nice to quote the Mfg. However if you have a top feul dragster DOT 5 would work well.

[christod]

I'm a new member and new to RV ing. I have a HMC 36 Ltd Edition with w cat 3116 and an Allison 3060 transmission. When decending a steep grade I use a switch on the dash which puts the transmission into a "braking" mode, so that if i take my foot off the accelerator, the transmission starts to downshift. Obviously as it does this the revs shoot up. I am concerned about this for fear of over-reving the engine, so I use my service brakes to slow the coach to lower the revs. Should I bother to do this or just let the transmission slow the coach and not worry about the high (2500) revs?

[wolfe10]

Christod,

Your 3116 has a governed RPM of 2600. 2500 is toward the high end, but certainly not over the redline.

I would suspect that your Allison is programmed to NOT allow engine RPM to exceed the 2600 figure.

Are you sure you don't have an exhaust brake? If not, what is your engine serial number. Beginning with engines produced the fall of 1992, the Caterpillar 3116 was fit with stronger exhaust valve springs that allowed the use of a very good exhaust brake.

Generally, the exhaust brake and transmission downshifting work together when the exhaust brake switch is turned on and the throttle is closed.

[hermanmullins]

Brett, does the 3116 have a Jake Brake or Pac Brake. If I understand, the Jake Brake is in the engine and controls the valve train while the Pac Brake is an exhaust brake that controls the back pressure in the exhaust. When I use my Jake brake my transmission drops to 2 and returns to 6 when I release it. I'm sorry that I don't know enough about the systems.

Since the subject is on Braking, could you give a brief lesson on the types of Exhaust Braking?

Regards

[wolfe10]

Herman,

The Caterpillar 3116 can be equipped with an exhaust brake, not an engine compression brake (same as Cummins B and C engines-- only large-displacement engines are generally fit with engine compression brakes). Prior to engine production of fall 1992, valve springs would only accommodate a low pressure exhaust brake. After that, they would accommodate a much more robust exhaust brake. Same thing occurred over at Cummins at this same time. That is why I asked for engine serial number.

The exhaust brake is wired through the Allison TCM so that when the exhaust brake switch is ON and throttle closed, two things happen. The transmission begins shifting TOWARD, REPEAT TOWARD the pre-select gear (generally 2nd or 4th but can be any gear you want-- I had mine changed to 5th). It drops one gear at a time as soon as it would not overspeed the engine in that next lower gear. At the same time, the butterfly in the exhaust brake closes, creating back pressure that the engine must work against-- kind of like stuffing a giant potato in the tail pipe.

If, for example you have a 2nd gear pre-select, it does NOT downshift to 2nd if you are at highway speed. And, if you are at a stoplight and look at the shift pad and it says "6" you are not in 6th gear, that is the gear the transmission will shift to as conditions permit.

Here is another discussion on this subject: http://community.fmca.com/topic/665-exhaust-brake-vs-retarder/

Brett

[hermanmullins]

Thanks Brett. That helps me understand and should help alot of other folks.

[christod]

My 3116ATAAC does not have an exhaust brake. ( I would like one and its on my wish list)

If I understand Brett's first answer correctly, in descending a grade, he is suggesting that I should engage my Transmission braking mode and let the engine rev up as it downshifts with the revs controlled by both the governor and the transmission and not try to lower the rpm's and vehicle speed with my service brakes. Is this correct?

[wolfe10]

Christod,

What is your engine serial number? I can call my contact at Caterpillar Corp to determine whether you can fit an effective exhaust brake (i.e. high back pressure brake) without fitting the stronger valve springs.

And, yes, to descend a hill with any rig-- gas or diesel, use the transmission to gear down to a gear that holds your speed in equilibrium.

If you don't have an exhaust brake on your diesel, you can do this (though far less effectively) by using the down arrow on your Allison shift pad. If you can fit an exhaust brake (depends on engine serial number), particularly a PacBrake PRXB, it is VERY effective.

And, I will repeat-- the safe speed of descent is that speed where you are neither speeding up nor slowing down on a long descent-- I call it your equilibrium speed. It is often NOT related to whether the road is straight or with lots of curves. A good reference is 18 wheelers. Your equilibrium speed will be faster than a loaded 18 wheeler and slower than an empty one.

[dickandlois]

Hi Brett, The use of brakes and engine exhaust assisted braking has been well covered in this thread. I would like to mention one thought.

When you have descended a long grade using air brakes and they have gotten very hot, it is always a good idea to let them cool down. Not that you have to stop,but if the road has more down grades its good to cool them down.Then you could feel like you need a brake to free your hands from the steering wheel and want time to reflect on the last few feet covered.

When you stop with hot brakes, pull on the parking brakes and place some tire chocks. Then release the park brakes.

The reason being,it they got that hot, as the drums cool down they will shrink and the brake shoes can restrict this action and it is possible to crack a brake drum. This does take a long time. However, if one takes an extended brake from driving before hitting the road again the possibility is there.

What are your thoughts? R.M.

[wolfe10]

****,

We have driven some really steep grades throughout the U.S, Canada and Mexico.

I have ONLY used the service brakes to slow down enough to "grab" the next lower gear. Yes, on one really steep descent in the Green Mountains of Vermont, that was 2nd gear with the exhaust brake on. No service braking needed. 15-20 MPH all the way down. We rounded a corner and "blasted by" a loaded logging truck, whose equilibrium speed was about 6 MPH.

Yes, I have seen and even smelled coaches where the driver did not use their transmissions, engine brakes and service brakes properly.

Brett

[planocat]

It is my understanding that the term "Jake Brake" comes from the original Jacobs Drive Line Retarder system that I had on my old Bluebird front engine diesel. It was about a $6,000 option in the early 80's and encased the driveshaft with an electronic magnetic field that, when applied, retarded the driveshaft from turning.

It saved my butt when after having the turbo rebuilt at the local Caterpillar dealer, the mechanic put the asbestos shield for the turbo on the wrong side. Going up Rabbit Ears Pass in Colorado the turbo got so hot it burned completely through the steel braided air line running from the compressor at the front of the engine to the air reservoir for the brakes. As we descended the pass, I noticed the air brakes weren't building back up. I used the Jake brake exclusively to come down off the mountain which allowed me to maintain the air level I had. When I got to the bottom, I told the wife and 4 kids that we weren't going anywhere until we found the problem and had it fixed. That was the most white-knuckled motorhome ride I have had since I started this motorhome madness in the mid-70's.

As an aside, Bluebird wanted $350 plus shipping for the replacement line. The mechanic said it was the same type of lines used in farming equipment. He took the old one and had it built locally for $65, put the heatshield on properly, and I never had another minute's problem with that system. So, that's a story on the original Jake Brake....... sorry it took so long to tell.